I spent about 2 hours this morning banging away at the BAA site to get my dad signed up. He was at work, so I did it for him.
I think a lot of this had to do with self-fulfilling prophecy. You hear about it in stock-market panics, and in other economic markets that are volatile. A rumor spread that the price of a stock is going to plummet, so people panic and rush to sell their shares... causing the stock price to plummet (thus fulfilling the prophecy). Sometimes the rumor of a price drop is the only thing that causes one, even if there are no intrinsic, concrete market factors to cause one.
I think that's what happened here. Everyone heard it would fill up fast, so everyone decided to get on the BAA website and sign up early... which caused it to fill up really fast, thus fulfilling the prophecy.
A second point: this is unrelated to how many people decided to sign up on day one, but apparently there was some type of internal hosting or link error that misdirected people once they hit the 'submit' button on the registration form. I had to re-fill out the form and re-submit it probably 40-60 times before it went through, and I read something that this was partially due to internal problems with the BAA site- it wasn't just server traffic overload causing the problem. So the BAA really f***ed up on that one.
And I also read that it affected some people and not others. I went straight to the BAA site, but I heard those using the link from Runners World didn't experience the problem.