Trying to download iOS 5 today seems like a guaranteed exercise in frustration.
The way I look at it, I've managed to wait four months already for iOS 5. Its not going to kill me to wait a few more days, or at least until the load on Apple's servers has reached manageable proportions.
There are probably a couple hundred million iOS devices extant in the world. And unless their owners have been living under a rock, they all know that iOS 5 is coming out today. Add into the mix that several key iOS Apps need to be updated, and you've got the recipe for extended downloading times.