Tonight my guild got its first attempts in on the Blood Prince Council. I was specced for tanking Keleseth as suggested, but had some trouble because I occasionally took melee blows from him and promptly died. I was wondering if any warlocks out there that successfully executed ranged tanking Keleseth could offer a few tips. The main problem I experienced was the fact that whilst running around collecting Dark Nuclei and experiencing the slow effect, he caught up to me and bashed me pretty hard. Do you guys stay on the outskirts of the room all the time?

I tanked Keleseth as 0/56/15 once we had a wipe working out the gimmicks.

He does follow you round the room however he won't run into melee range of you by himself. However, if you run into his melee range he will melee you and probably one shot you.

The debuff from the Nuclei is multiplicative not additive so at 3 stacks you are taking 27% shadow damage, 4 stacks 18% and so on down to 5% at 7 stacks (the most I managed to get). On top of this damage reduction various warlock talents can reduce damage taken it further (Molten Skin, Soul Link, Nether Protection, Master Demonologist) as well as Shadow Ward obviously.

I found myself running around the room a lot however I think a better strategy would be to make liberal use of misdirects and tricks to get the farther Nuclei onto you. Despite the running around and lower personal threat due to being Demo I had no threat issues at all, almost certainly because DPS would only spend 1/3rd of their time on Keleseth. So really, any spec of warlock will work fine for tanking, damage reduction or stamina talents HELP (especially Soul Link) but aren't essential. If it's not going to be a warlock your best other choices would probably be a Shadow Priest or Hunter (in that order) rather than mage/moonkin/elemental.

Getting the achievement for the encounter hinges on having plenty of Nuclei stacks for when Keleseth is empowered . Use any absorb cooldowns you have available on the first one as it can be difficult to have 4+ that early in the fight. After that just focus on keeping as many Nuclei as possible and you should get it.

What slow effect is this?

I did not see any slow effect in 10 man. It was pretty easy to run around and collect basically every single nucleus on the map.

The 20% slow from glittering sparks, perhaps? Granted, that shouldn't be on anyone but the Taldaram tank if things are going well, but it's easy to get if you don't know beforehand to avoid it.

And to Epoché, Keleseth seems to prefer casting to melee, as the only time he hit me was when I ran into his melee range thanks to the six huge balloons clogging up my screen. So if you didn't accidentally do that, I'm not sure what the problem might have been. His shadow lances aren't interruptable, are they?

Our demonology warlock tank also died to melee hits when trying to collect Dark Nuclei. After that, we had him stand in the middle of the room.

I asked a hunter to MD the nuclei to me and it didn't work. Obviously he could have just messed up, but it seems that the nuclei dont respond to MD so we used them on Keleseth instead to help me hold aggro while I was running around the room.

We managed the ten man achievement (Orb whisperer) in my guild run yesterday and I can say that MD didn't work for us either, we also used it to help me hold aggro of Keleseth while it was the target for the rest of the dps.
I had between 6-7 nuclei on me the whole time (except for at the start as they were building up) and as said the main problem was all the moving around. Metamorphisis charge and summoning portal are very useful for this fight as a warlock tank.

Melee hits will, indeed, one shot you. Usually around 30-35k.

And once again, you do not need a specific spec or glyphs to tank this fight, I did it fine in 0/56/15.

You should be able to collect the orbs yourself and not waste another player's dps. Just have your base position fairly in the center and then be conscientious of where your healer is.

Also make sure to tell people not to heart strike, cleave, searing totem, maul glyph, etc if they get close to orbs.

Anyway, I don't think a warlock is that much better of a tank for this than other classes. DKs have the health pool, the cooldowns, and enough ranged attacks to collect orbs himself. Only downside may be threat. Moonkin, Shadowpriest, Mage, and others can tank as well.

Certainly others could tank, but the 0/56/15 standard raiding spec warlock really would be the simplest option (w/o respecing) due to searing pain threat modifiers, and soul link and felhunter combined 30% spell damage reduction.

On a side note, this is the action priority list I would advocate:
(1) Gain decent threat lead in first 20 seconds
(2) Stay out of melee range
(3) Gather ALL orbs you can reasonably get to (< 3-5 seconds run)
(3) Avoid flames and vortexs
(4) Damage on real attack target

I tanked him last night as 0/56/15, i had my felpuppy out and just set him on him as we fought. all i did was CoE him and Searing Pain to pull. the rest of the fight id use a searing pain to pull The orbs to me and any other raid members ran to me, and to hold aggro just hit him with one SB than spammed Soul Fire for threat worked out perfect. it took me a couple wipes to get this down but on the 4th try we got him down fairly easy.

I have not used searing pain for threat on anything in wotlk, which is pretty much Mimiron, the Jormungar twin, and this (and some joke attempts on bosses where I'm trying to pull off tanks). Conflag gives the best early threat, and you get a ton more damage using the regular rotation.