Sounds like the servers are busy. I don't know if some kind of sale is going on or not (I'm at work and cannot get to Steam from there).
I do know a few weeks back when they were having their uber-sale going on, there were times where trying to download a purchase was next to impossible because of how many people were hitting the servers due to the sales.
I say keep trying. Eventually you'll get through and this is an exception, not a norm with Steam. A very RARE exception.