According to this blog it looks like Windows 7 is going to include native support for mounting VHD (virtual hard disk) files. The information comes from a job posting at MS, which states:
This will also be a fantastic development for anyone that uses VHD files for virtualization (i.e. MS Virtual PC), which is going to be a much bigger part of W7.
If that means we can have completely different boot configs, without the need for dual booting then this will be a massive development leap.Imagine being able to mount a VHD on any Windows machine, do some offline servicing and then boot from that same VHD. Or perhaps, taking an existing VHD you currently use within Virtual Server and boost performance by booting natively from it.
This will also be a fantastic development for anyone that uses VHD files for virtualization (i.e. MS Virtual PC), which is going to be a much bigger part of W7.