Have just noticed that setting acpi=off in the kernel boot options seems to make X work. It did not, however, work on the system I'd upgraded from Dapper to Edgy. This could be because of some pre-Edgy cruft or it may be that an update to a package in the 3-4 days since I last tried has made X at least work with ACPI disabled.
I had noticed that on booting I got a dozen or so messages on the system console which look possibly ACPI-related, like the below example : Calling INT 0x15 (F000:F05E) EAX is 0x5F03 -- X server fails after upgrade Dapper > Edgy https://launchpad.net/bugs/58326 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
