The integration of Wolfram|Alpha seems to be one of the (many!) major features of Mathematica 8. Earlier I was reading this Wolfram Blog archive and noticed quite a few articles talking about the use of this API.
But what finally got me to ask my first Stack Overflow question was reading the top ranked answer on this question which uses the programmatic interface to Wolfram|Alpha. There is, to my knowledge, no published information on how much Mathematica users are allowed to use this service.