Here Come The Tweets: Gnip Offers Access to Full Twitter Archive

I'm curious why Twitter are OK with this: it seems like a core bit of functionality for them to control. Is there a backstory to Gnip? This is the first I'd heard of them.

Are there cost estimates for running this sort of search?

I thought it was awesome until I read "publically" on their site, and then clicked away. If they can overlook a spelling mistake on their website, how can they be trusted to archive PUBLICLY available tweet since 2006?