One way to make sure that is not the case is to disable barriers in the guest. Meaning in /etc/fstab have something like this: /dev/xvdc /blah ext4 errors=remount-ro,barrier=0 0 1
That seems to fix it. It was remounting as read only either during the boot process or immediately after, and now it boots up and seems to stay up. I'll test laster with a DomU that actually has things running.
This also fixes the reboot problem I noted earlier, init 6 now reboots the DomU rather than destory it.
The other question is what version of Dom0 are you running? Is it 2.6.32? 2.6.39?
squeeze, running linux-image-2.6.32-5-xen-amd64 2.6.32-35