First, thanks for the help!
Reading your commenct I decided to re-check the mtrr settings and found the following:
mtrr: your CPUs had inconsistent fixed MTRR settings
mtrr: probably your BIOS does not setup all CPUs.
mtrr: corrected configuration.
Will rewriting the mtrr settings using the /proc interface solve the problem?
$echo "reg01: base=0xb8000000 size=0x10000000 type=write-combining" > /proc/mtrr
(Hardcoding the driver to detect 256MB seems like a lousy idea...)