I have successfully upgraded my ProxMox production system to Squeeze repo. Nothing awful has happened. But, if you decide to reproduce my experience, you must apply you head brain to process.
1. First of all, try to perform "dist-upgrade" action on a test system, not on production one.
2. BEFORE upgrade, freeze (hold) the following list of packages (if you have such installed): cifs-utils, linux-image-*, pve-kernel-*, samba, samba-common, smbfs, vzctl. For example, you can do this such way as "echo "vzctl hold" | dpkg --set-selections", etc. Mentioned packages should not be updated, otherwise your system will be broken.
3. Before upgrade, manually update packages "locales", "mysql-server" (if you have them installed) from Squeeze repo.
4. You may meet conflict situation with the "pve-firmware" package, as it interferes with squeeze's one. I have resolved this trouble by accurate removal of "pve-firmware" and its dependencies. But you need to be very careful in order not to purge anything really necessary with it.
After satisfying this prerequisites you can safely upgrade your ProxMox installation up to Squeeze release, if you'd like to.