Hey, Kevin,
Until very recently, I'd been running the 2005 beta using VMWare, because of several recommendations. No one had ever mentioned a specific reason, just that it could cause problems with 2003 stuff.
I decided to take the plunge, mainly because I'd spoken with so many developers who were working with both. I haven't had a problem.
BTW, I have an article in the brand new issue of CoDe (Nov/Dec '05 issue), just now hitting the stands....it's "13 reasons to upgrade to VS2005".
Kevin