There's a very strong hint on the site. If you're a developer you should be able to figure it out easily, if you're not a developer you probably won't know how to "remove the disabled attribute" anyway ;)
All it takes is someone to post a few step by step instructions and anyone could re-enable the button regardless of their skills. Hopefully people won't ruin this offer and do that though.
I'm pleased to see that slickdeals and fatwallet both have threads about this deal, but no one has spoiled the puzzle yet.
Or a Chrome plugin :)
It is easy enough to write a small bookmarklet that anyone can paste into the window to flip off the disabled field. Once that gets into the wild, I can't imagine this offer will last.
Fortunately, it sold out before that became a problem.
Lanthe, how much longer do you intend to keep this running? Because I'm timing out before even hitting the form. Would really love to get this as I've been hoping to get into .NET MVC4 but OEM Win8 pro is quite a lot of money.