With the release of Exchange Server 2003 SP2 we now support Exchange running on Virtual Server 2005 R2 release. This means you will have to be using Virtual Server 2005 R2 to get this support. The last I heard is we are 3 weeks away from shipping the R2 version of Virtual Server so this combination of products will be realised very soon.
The formal policy is now stated here.
It’s probably worth a quick review of Microsoft’s overall support policy on virtualization offerings. Traditionally, such support from most vendors has required that any problems be reproduced on a real (not virtual) machine and that is still Microsoft’s policy on non-Microsoft virtualization environments (unless you are willing to pay for Premier Support).
However, with the advent of Microsoft’s Common Engineering Roadmap, support for all Microsoft Server products on Microsoft Virtual Server has been mandated, but some products are still working on achieving it. This announcement is that Exchange is moving into the compliant column.