I had some troubles with removing an LVM snapshot from within a Debian Sarge domU. It would lock up the domU completely, without removing the snapshot. A bit of a problem, especially since this was a production machine. So I destroyed the domU and tried again, with the same results, unfortunately.
Bart found a description of the problem and the solution at Jürgen Kreileder's blog. Jürgen apparantly still has the repos online, which helped us resolve the problem on our machine. Much appreciated!