I've recieved a couple PM's on it, so I've decided to post my shadow priest talent guide.
NB: Disclaimer, this is for a PvE build primarily. You want PvP look to someone else. I barely PvP on my Paladin, why would I want to PvP on a class that has less armor, less stamina and who's very existence is a red flag to the Horde's bull? ;)
Early levels: Wand Spec is tempting, but to get it you're holding yourself back from getting Mind Flay for five levels (unless you want to respec at 20), so I'd recommend early shadow all the way. First five points into Spirit Tap, then get Imp. SW:P and 3/5 Shadow Focus. Shadow Affinity is nice, but that's primarily a grouping tool so you don't pull aggro when you're DPS'ng, and most times if you're in a group at that level you're not DPS'ng. Blackout is another discussion, but I find it unreliable for use while soloing, and generally a distraction when it does happen. For PvE soloing, Spirit Tap > Blackout by a wide margin.
Level 20. From here, I'd recommend two points in Imp. Psy Scream (to prep for Silence later and also for the shorter cooldown, fear is your friend when you wear cloth and your main armor source has charges). What to do with the other two points at this level is up to you, personally I'd finish out Shadow Focus, but Imp MB is a valid choice*, as is Blackout if you like your stuns. At 25 get Shadow reach, then 3/5 Shadow Weaving, then VE.
* Imp MB seems nice because MB is your only direct nuke. However, Mind Blast has shocking mana efficiency, and so I tend to use it only to pull (front-loading damage means basically the mob starts with less health, which is always good) and if I really need to ramp up DPS. Otherwise, I stick with mindflay/pain/VE/VT. Boring, yes. Effective, yes.
Level 30. From here we have 13 very good talent points to go into nine spaces to get to 40 - the final 2 Weaving, 2 Imp. VE, 1 Silence, 5 Darkness, 3 Focused Mind. Imp. Fade is just a joke, I've *never* met a Priest who went shadow and specced Imp. Fade.
For myself I'd choose to max up Weaving (having a 60% chance to proc debuff decide not to proc four times in a row is just annoying), grab Silence (utility value is just too high, from silencing healers trying to heal themselves to pulling casters away from adds), get 3/3 Focused Mind (more mana is always good, and this is about the only option you have at this point to get or keep mana), and 2/5 Darkness, then the big kahuna, Shadowform.
Level 40. At this point, options are fairly slim. Shadow Resilience is junk for PvE grinding (mob spells can't crit), and Shadow Power doesn't come into it's own until you have Shadow Word: Suicide (SW: Death) to play with. I'd recommend filling out the remainder of Darkness and grabbing imp VE. Getting Blackout, Shadow Affinity or imp MB are also options ... mix to suit. Imp VE means more health back to you, Darkness is more damage, whereas the other talents are utility mostly. At 45, 5/5 Misery, then at 50 VT.
Level 50. So now you have all the meaty talents, where to next? I'd recommend getting 2/2 Healing Focus, mostly for instances with learner tanks (and for warriors at least, even good ones sometimes have issues holding every mob in a big pull). If you catch too much heat soloing, Fear+Shield+flee is generally a better option than trying to healtank a mob, so that's why I didn't go for HF earlier.
From here, we go for Disc up to Meditation/Inner Focus. In Tier 1, Wand Focus vs. Unbreakable Will is user preference, both are fairly marginal - by the time you're 30 and definitely by the time you're 50 you shouldn't be wanding except for when you're waiting for the mob to die or if you're OOM.
Tier 2 Disc is also more user preference, you can grab Imp Fort if you're instancing quite a bit, Imp. Shield if you use that quite a lot, or whatever takes your fancy. Personally I'd go for Silent Resolve (I don't pvp remember), since that means you're less likely to be squished in an instance.
Tier 3 Disc: Inner Focus/Meditation. These two should speak for themselves and be fairly obvious. Absolution? Meh, you're not going to be dispelling/curing *that* often, and if you are, reroll paladin. :p
Level 66. From here we have four points to spare, I'd probably toss them into Shadow Power (more crit = more damage).
Level 70 Solo Priest spec.
That's also remarkably close to my
Level 70 DPS priest spec, the key difference is less power more affinity. Having both threat reducers maxed means you do 80% threat for holy spells and 60% threat for shadow spells. If you want to do comparable damage to any other DPS class and not be a squishy spot on the floor, you're going to need all the threat reduction you can get.
Little mental exercise here - assuming your group is warrior/rogue/spriest/(two mana users to suit), and that 50% of VE ticks are non-overhealing, and that none of your VT goes to waste on the three characters it applies to. If you do 1k damage with 0 reduction talents, you'll get 1000 threat from damage, 1000*0.3*0.5*0.5*5 (30% of damage goes to VE, half threat mod for healing, half of VE ticks are wasted, VE affects 5 people), and 1000*0.05*0.5*3 threat from VT.
[I wouldn't be surprised if my numbers are slightly wrong, but the whole point of this divergence is to show that shadow DPS priests have significant threat from sources aside from their direct damage, and as such need to take more care than most DPS classes do with regard to threat. These numbers show that fairly well. Incidentally, if anyone cares to correct the numbers for me, feel free.]
That means for 1k threat from damage, with VE/VT up you get 375 threat from VE and 75 from VT. So for 1k damage you get 1450 threat. Too late in the day for me to factor in threat reduction and figure out how much threat your talents will save you, but I can tell you without any threat reduction you generate 45% more threat. In a threat-sensitive fight that means you can only do 69% of the raw DPS of a mage/lock (1/1.45 = 0.6897).
[edit: two questions here ... since VE is a Shadow spell, does that mean the health return is covered as Shadow and as such gets 40% reduction (from Silent Resolve/Shadow Affinity) in threat? Also, is VE threat spread among all the mobs in a pull like healing threat is, or is it single-target threat only?]
PS: If I ever decided to pvp as a shadow spec on my priest, I'd probably go something like
this. A lot of the group-friendly and mana-endurance talents go by the wayside, in favor of raw damage and some survivability. Levelling with that build would be a little bit slower than a PvE-optimised build, but not crippingly slow (primary difference is how often you'd need to drink).
edit: typo, "would" instead of "wouldn't" completely changes the meaning of a sentence. And oops, repetition makes me sound dumb.