Before I upgraded my dom0 to Fedora 16 (and by association Grub2), I had the dom0 memory limit fixed as an argument in grub.conf. When I upgraded to Grub2, I initially added it manually to the Xen boot command line, but eventually stopped doing that.
Today while looking through the xen-users list, I found an email that mentioned how to get grub2-mkconfig to automatically add it for you – add GRUB_CMDLINE_XEN=”dom0_mem=1024M” to /etc/default/grub. It seems to be the same on both Debian and Fedora, so I don’t see why it wouldn’t work on dom0s built with other distros.