Virtual PC futures have been a little quiet in terms of announcements. There will be more shortly, but if you weren't already aware, Virtual PC Express will be out sometime around March next year. Virtual PC Express is a benefit for SA (Software Assurance) customers only, and the release will co-incide with SA 3.0. Virtual PC Express is a fully featured version of Virtual PC 2004, but with the single exception of only being able to run one virtual machine at a time. Other management functions including creating and re-configuring Virtual Machines will be available.
In addition, when Windows Vista Enterprise ships (ie client - not Windows "Longhorn" Server virtualisation), there will be a version of Virtual PC Express which will be validated on Windows Vista, and will include additions optimised for Windows Vista to run as a guest. This version will support x64 hosts, and will include a number of additional performance enhancements.