version checkpointing

[Gregory Seidman <gss+debian@cs.brown.edu>]

With some regularity, I want to try some package from unstable yet be able
to undo all changes back to some previous state. I'd like to be able to
checkpoint the system (which may require saving .deb files) and be able to
return to the checkpoint. Ideally, a checkpoint would be "open" in that it
would not be just a backup of the system (I could do that with a tape drive
and dump), but actually add .debs to the checkpoint that had been removed
or upgraded by subsequent apt-get runs.

Is this available? Is there a good workaround? Is anyone interested in
implementing it if there is neither an implementation or workaround?

--Greg