I would like to add that Andrey Mirkin has presented a paper on checkpointing and live
migration during the last Linux Symposium. The paper, available at
http://ols.fedoraproject.org/OLS/Reprints-2008/mirkin-rep... (~200K PDF), goes into some
details on how CPT is implemented in OpenVZ. It also lists a few other existing checkpointing
implementations -- some were already mentioned in the above comments.