I can confirm this behaviour. The machine just freezes when i try to save it and has to be destroyed to get it back. This happens to a Debian Lenny x86 DomU with 2.6.26-2-xen running on top of a x86_64 Ubuntu Hardy Dom0 (2.6.24-24-xen), so i guess this isn't a Debian specific issue but a general x86_64/ x86 related XEN problem.