Couple of things here...First, the ASTB, regardless of it being old or new is good for life. There are a lot of rumors that it is good for five years or whatever, but that's all trash. It's good for life, you can retake it the first time after 30 days and each subsequent time every 180 days.
As for a rolling selection process, that is somewhat true. You didn't mention if you were looking at the USMC Option or were going USN. For the USMC Option you can put in a 'package' through your MOI and a board - basically a an O-3 who does this stuff for HQ USMC - will make the determination. This usually takes a few weeks...I've seen it done three times this year and it took an average of three weeks, FYI.
If you are staying USN, then you have to go through the Service Selection process just like everyone else. Please see the various and sundry threads on the topic for an in-depth analysis of the process. But here is a cursory view...For December grads (like me) you submit your Service Selection (SS) package at the end of the summer before you graduate. For me, it was due on 01 AUG 04 and the board supposedly convened sometime after 18 Aug 04. That 'supposed' date is really flexible apparently as evidenced by the fact that those of us waiting on those results have not found out yet (hurricane didn't help either!)
For Spring or Summer grads, the SS package is due in October of your Senior year. So, for Spring/Summer 2005 grads, that would be in 19 days. The boards will convene shortly thereafter and you'll find out when you find out. I can say from last years Spring grads, they found out literally on New Years Eve. Good night from what I hear!
Hope this helps. If I'm off-base I trust my fellow Airwarriors to correct me.