I'm running into a similar issue with a Debian6 system fully updated (standard and -updates repos).
It's an IBM x3850, 16G of ram
I tried setting max_dom0_mem like stated here  but didn't solve the issue, the server reboots after scrubbing free ram. Still hadn't chance to connect a serial console.
I also tried setting:
GRUB_CMDLINE_XEN="dom0_mem=512M dom0_max_vcpus=1 dom0_vcpus_pin"
but no go.
Any other possible workaround?
I few months ago I installed two other servers with Debian6 and much more ram and didn't face the issue.
Lorenzo Milesi - firstname.lastname@example.org
GPG/PGP Key-Id: 0xE704E230 - http://keyserver.linux.it