from the release notes for FreeBSD 6.0 "cdboot now works around a BIOS problem observed on some systems when booting from USB CDROM drives."
sweeeeet. This was kicking my butt when I first started using pfSense. I was really hesitant about upgrading my FWs because if they got horked really bad the only option was opening up the case and jumping through hoops. Hopefully this will resolve those issues once pfSense starts using 6.0 (if it isn't already). -Matt