No thank you. As someone who has to read code in addition to writing it, it's great to have the doc comments right there, so I don't have to open and search through another document.

There's also the issue of keeping things up to date: it's much more likely that a programmer is going to keep docs up to date if the doc is right there along with the code than if it's in a separate file.

Not to mention... XML... ugh, no, let's not.