This is possibly big news, something I've hoped they would do for a long time and something Fred Wilson voiced his support for a few weeks ago: Creating user accounts via the twitter API.
Both the projects I'm working on full time could benefit from this. Sproozi of course because we could create user accounts for them and help users find interesting local people to follow. My oft promised side project which at it's most basic level [the one I'll launch first] aims to make making announcements easier could benefit by making the signup process seamless - users wouldn't have to already have to have a twitter account, they could create it all in one step with us.
That I think is what makes this such big news, sites that rely on users already having a twitter account to do something useful will be able to create that account as a seamless on site process. That will help drive users to the sites and drive users to twitter. Let's hope they start rolling it out to other developers!