@jimp:
Try the fdisk/bsdlabel suggestion here:
http://doc.pfsense.org/index.php/Boot_Troubleshooting
Should also work when booted from a 1.2.2 CD.
If you can wipe/reformat, you might also want to make a small / partition (less than 4GB) and make the rest /usr - obviously that requires a custom install and not a quick/easy install
Jimp, I also tried booting from a 1.2.3 livecd and even the livecd doesn't load with the same error message. From http://lists.freebsd.org/pipermail/freebsd-stable/2009-December/thread.html#53132 , they seem to be saying its a memory issue with the loader and a possible solution is offered.