This got somewhat off-topic. I have a few comments on-topic:
First, the offer is legit. I got two emails directly from Microsoft on the issue. So, any of you that were worried shouldn't be.
Second, I'm going to try this, although I don't technically fall into the correct timeframe.
Third, those of you getting blank screens aren't alone. I too, am having the same problem. And I think it is because their system is getting hit incredibly hard. I received the same screen a few nights ago when signing up for the Halo 3 beta. Methinks when that dies down, the site will work again.
Oh, and the emails I got had codes "XBXNG2" for three free XBLA games and "XBXNG4" for two free XBLA games.