It depends if Ulduar will bring us one, two or three viable trinkets. But also consider the fact that you won't be alone going after these trinkets (even if there will be two), so investing in a Greatness card will mean that it will last at least in one slot for quite some time.
Then again with DMF over, you might want to think about it.
Next DMF is up in a month and who knows, maybe we'll see Ulduar earlier than that.

Yesterday I picked up a Mark of Norgannon in hopes of a possible buff in 3.1 or maybe stacking haste will be the new premiere stat. Right now is it the most lackluster 226 item

Anyone else have thoughts on haste in 3.1?

Darkmoon cards weren't very good for Rogues because we had access to the DST, along with a relatively overpowered T5 Class trinket. The closest analogue, the Blessings Deck, was far better for caster dps than us, and it did in fact last many of them well into BT.

None of the announced changes affect the relative value of Haste in a positive direction.

This will depend entirely on Blizzard. Prior to this, darkmoon cards were never very good for us. One of the first first trinket available in BC was [Dragonspine Trophy] from Gruul, and people were still using it in Sunwell. If Blizzard gives us a better trinket we will use it, if they dont it will last a while. Your guess is probably as good as anyones at this point in time.

With the changes expected in 3.1 and the current changes in 3.9, are T7 Hat/Legs/Hands/Shoulders and best-in-slot Chest still considered best for mutilate?

Yes, neither Frosted Adroit nor Leggings of the Honored won't become better.

I have a question about combat post-3.0.9. I've seen a lot of numbers flying around about BiS and ideal combat setups (where before it was usually adroit, 4/5, thrusting, etc) and most of the spreadsheeted numbers seem low. As an orc rogue in BiS (alch/jc) I managed to peel 5651 dps out of the spreadsheet running 2s/5r/5e, CG/WD, adroit, thrusting, and grim toll and this is with minimal tinkering.

Is it possible that people are still running with 7/51/13 spec? 15/51/5 already eeked slightly ahead of serrated in BiS gear before, and after the SnD buff I should think it would be obvious that lethality would win. Faster attacks->more value for hit as potential poison dps->more combat potency (for both reasons)->more SS.

Net effect on rupture dps from 3.0.9=zilch. Net effect on poison dps (ignores armor component of serrated) and energy gain=pretty substantial.

Maybe I'm off-base with this, but it seems like the majority of the low numbers (people quoting ~5250-5400 as BiS combat dps) are coming from mut rogues who may be modeling off old combat theory. It should probably be more explicit amongst the community that 7/51/13 will no longer yield the best returns.

Edit: clarifying why I posted rather than just babbling.

What's your basis for that assertion? Conventional wisdom says otherwise. Yes, the ability to time cooldowns against class cooldowns is useful and increases the value of these trinkets - but they are so far behind to start with, they are generally regarded to be inferior anyways. For instance, Blade Flurry increases your attack speed by 20%. This equates to, say, 15% more DPS while it's active - probably less, actually, but lets go with 15%. Thus, over a 2 min period, despite the fact that your Blade Flurry is only active for 12.5% of the time, you actually do 14.1% of your total DPS during it; thus, the value of the AP cooldown on Loatheb's Shadow is, to a first approximation, 122 AP assuming it is always used overlapping with Blade Flurry. Thus, it's better than Mirror of Truth only if your Mirror uptime is less than 12% - and in practice, Mirror uptime is 17-19%. Stacking AR and/or Killing Spree onto Loatheb's will increase it's value somewhat more, but it's nowhere near enough to make up the difference between it and Mirror, much less it and Greatness. Fundamentally: while cooldowns do give a significant damage boost while they're up, they account for a relatively small portion of your overall damage, and, as such, don't increase the value of activated trinkets enough to catch the proc trinkets.

Thus - unless you have some evidence, testing, or theorycraft that says otherwise - it's somewhat misleading to assert that MoN and LS are the best trinkets for you as combat, as all existing data indicates that Greatness, Mirror, and FotFF are all superior to both LS and MoN.

Just to be completely explicit - on the side of people who think that stacking cooldowns does not magically make trinkets like LS and MoN better than their commonly-accepted value, we have as evidence a spreadsheet that actually models the exact implications of the ability to stack cooldowns. I would recommend having at least as much evidence on your side before you try to pass such assertions as fact.

Not to mention, you can always watch your buffs and use Flurry / Haste pot / etc while the trinket proc is up. It should generally line up at least partially with Hero/Lust.

You are correct that Killing Spree should not be used during AR, but I think what Aldriana meant was just about using AR and KS while buffed by Loatheb's trinket and not with each other.