The SPAZ guys posted a clarification. The Steam activation was meant to be a bonus for people who purchased the beta, so people will need to buy from Steam if they want the game on Steam.
It's not totally clear from the thread, but it sounds like they might be transferring keys to Steam for non-Steam purchases made yesterday, though.
Pah, bastards. I just got it on GMG this morning because it was said it still worked on recent purchases.