Yes I do think this will boost sign ups. It's free and Tweets, for better or for worse, have come to be a sort of "source material." I think many, particularly younger, news-savvy users will seek to access Twitter.

I also think it greatly boosts Twitter's ability to monetize advertising. That's what make Facebook and Google so valuable (just about everyone is logged into an account).

I am, however, curious how this will impact embedded tweets (in articles, for example).