The word on the street is that lots of people were disappointed about missing the beta for exam 70-620: Windows Vista, Configuring; the "big Vista" exam (in its beta state, the number is 71-620). We're sorry if you are one of those people--we'd love to have everyone interested get to participate but unfortunately we can't pull it off.
In the case of 620, there was so much interest in the beta for 620 that the seats filled up ridiculously fast. A lot of people missed out, including MSFT employees, MCTs and MCPs, MVPs, and some of our biggest customers (that's a lot of loud lobbying, fyi), so we opened up another set of seats. Those, too, filled up immediately.
Some background on betas, if you're interested:
Really good info on why/how we run betas here: http://vishaljoshi.blogspot.com/2005/11/microsoft-beta-exams.html