I wouldn't be surprised if Goldman and Citi merge. Goldman needs retail banking deposits, and now that Citi's mortgage portfolio losses have been backstopped by the FDIC, they become much more attractive. And there just aren't that many large banks with large retail bases around anymore. Citi needs credibility, and Goldman can provide that. Goldman's market capitalization is 25.7 billion, Citi's is 39.3, neither can swallow the other out of spare cash, but both are down far enough that they would see each other as good deals. I haven't heard this anywhere else, it's just a guess, but I have to think they're both considering it.
Addendum, 12/15/08: Goldman has reportedly looked closely at Citi but decided against a merger because of Citi's deep-rooted problems.