Microsoft has published a new FAQ on VHDs. There is some surprising information in here and it is very recent. There are guidelines for using VHDs with Windows 7.
It was unexpected advice to use Fixed VHDs. The reasons sound good but at the same time very wasteful. It might be more from the angle of supporting a local non-virtualized Windows. Dynamic VHDs would make more sense in a data centre where the power and disk space is highly managed.
I searched for subjects related to pagefiles and found this gem. The interesting news with mounted/booted VHD drives in Windows 7 is:
Are paging files supported in VHDs, and doesn’t that affect the performance of systems using native VHD boot?
Windows does not support locating paging files on virtual disks of attached VHDs. This would include pagefile.sys, hiberfile and crashdump files. Native VHD boot performance would not meet our system responsiveness goals if the paging file were located inside the VHD. If Windows 7 starts using native VHD boot, the operating system locates space on the host volume outside the VHD file for a paging file. The paging file can be approximately 2-4GB or more in size, depending on how much physical RAM memory is configured on the system. Plan the host volume free disk space to support the VHD file and paging file required for native VHD boot. If the host volume for the VHD does not have enough free space for a paging file, Windows attempts to locate the paging file on another volume with sufficient free space available. Note that when Windows is running in a Hyper-V virtual machine, a paging file is created inside the VHD because the virtual disk is used as a normal system volume.
VHDs seem to be gaining traction having been used extensively by Microsoft and also documented.