Yes, you can make your reference .vhd read-only and attach a Differencing Disk:
Differencing - Differencing is a
concept which involves a parent disk
and a child disk. The parent disk
contains a baseline hard disk image
with the operating system and perhaps
a range of applications and data
installed. Once this baseline parent
disk is configured, a differencing
disk is assigned as a child. When the
virtual machine to which the disk is
assigned runs, any changes made the
baseline parent image are actually
stored on the child differencing disk,
leaving the parent disk unaltered.
Multiple child disks may be assigned
to a parent disk allowing multiple
virtual machines to share the same
parent disk. Differencing disks may
also be chained together. The changes
contained in a differencing disk may
be merged into the parent disk.
Alternatively, a new virtual hard disk
may be created containing the merger
of the parent disk and the child disk,
thereby leaving the parent disk
unaltered. When creating a
differencing disk, the parent disk
will need to be specified. The size of
the differencing disk increases as
space is required. Differencing disks
may also be compacted to free up
unused space (see the section below
entitled Modifying Existing Hyper-V
Virtual Hard Disks).
This has a performance penalty though, like running off many parent snapshots, but it most probably will not be noticeable unless your server is very busy.
I cannot remember if you can set the VM to always start from the parent image, but I think you can. Someone must have made a PowerShell script to do it at worst.