I have a 2006 disc. I am thinking about buying the 2007 set since they are only $15 on ebay for copies or aftermarket (whatever they call them). But that would be a waste if the 08 disc will be out soon.

08 will be out sometime next year. Pioneer won't give a eta

IIRC the next generation of nav units should be debuting sometime in the spring, and I'm assuming they would be including a new 2008 version nav disk with them, not the current 2007 one. So I'd guess that the updated nav disks would be coming out around that time. Maybe a while afterwards, but it's doubtful it would be before the release of the new models.

I just ordered the discs from Pioneer and I asked when the new ones were coming out cause I didn't want to have to buy the 08s so soon. She said mid to late Summer is usually when the new disks come out.