According to this article:
http://www.topspeed.com/cars/porsche/porsche-boxster-ar26774.html
the 2008 Boxster will be revealed at the Frankfurt Auto Show. The show starts Sept 13. I have no idea if the article is accurate or not, but assuming it is, I would guess the "new" Boxster with changes would be available a few months later.
I can't imagine this article is referring to the 2009 model. That seams like it would be showing the model too early (I haven't followed Porsche long enough to know if they would do this). Either way, after they show the new Boxster with more HP and other improvements, the 2007 should be a little harder to sell -- and maybe better pricing.