"A Cornered Rat" doesn't start, and nothing I do can initiate the quest. I've already restored the thieves guild and everything. I even went to Esbern without quest markers and there's no dialogue options or anything.

Laaaaaaaaaaaaame.

Same thing happened to me. I am also leader of the Thieves Guild. I suggest continuing from a save you made before you turned in "Diplomatic Immunity" just in case. I'm actually going to continue playing from a save before I did "Diplomatic Immunity" for extra security.

Thing is, I've already pumped 11 more hours into the game since. Not doing all that again. I have patience...

Well, I'm not sure if a patch would save your main quest playthrough or not, but the good news is that I have seen this exact bug reported a number of times already so it will probably be one of the earlier quest bugs to be patched.

Also, almost every instance of this bug I have read the person was in or more commonly the leader of the Thieves Guild. I believe Delphine's dialogue is slightly different if you are leader of the Thieve's Guild. Normally she tells you to see Brynjolf, but if you are the leader of the Thieve's Guild she just tells you to ask around the Ragged Flagon. Maybe they forgot to test the quest trigger for the alternate dialogue?

whenever gamesas gets back from their vacation (we paid for)

Got the same issue, tried loading a previous save (just before Diplomatic immunity) didn't help.
Most people with the problem seem to be the leader of the thieves guild but im not, so it doesn't seem to be that but if any one finds a way around it please let me know thnks

Are you at least a member of the Thieves Guild already? I wasn't the leader at the time, but I done the "main" quest line for the Thieves Guild.

I definitely sense that it has something to do with Brynjolf(?) and one's current progress through the guild already.

I agree, its got to have something to do with him or at the very least the Thieves Guild questline.

how long does it normally take for a patch release?

Hopefully soon, eh!

I'm not a memebr of the thieves guild and it was glitched for me. my game's dead in the water....