That would be most excellent. In addition, it'd be really nice if TextMate had the option to look in other files in the project -- for example, the completion of a function from another file.
I have wanted this since TextMate was first introduced. It's probably my top wish to have!
I think I want it too, though not so badly, but there's a downside to it: you begin getting too many options. I had this problem with jedit all the time.
But if we make a trimmed list, with only the more common or long-named methods, not, for example, the entire ruby library, it'll certainly be helpful.