The LiveCD (memstick) image boots fine.

M0n0wall (bootlog) and FreeBSD 8.2 (bootlog) also boots fine.

Here is part of the boot log for the two pfSense images:

pfSense-2.0.1-RELEASE-4g-i386-nanobsd.img (bootlog)

uhub7: 3 ports with 3 removable, self powered
Root mount waiting for: usbus7
ugen7.2: <sandisk>at usbus7
umass0: <sandisk 0="" 2="" cruzer="" fit,="" class="" 0,="" rev="" 2.00="" 1.26,="" addr="">on usbus7
Root mount waiting for: usbus7
Trying to mount root from ufs:/dev/ufs/pfsense0
ROOT MOUNT ERROR:

[…]

mountroot> ?

List of GEOM managed disk devices:

[…]

–-----------------------------------------------------------------------------
Inserting 64 MB usb flash in external port
-------------------------------------------------------------------------------

da0 at umass-sim0 bus 0 scbus0 target 0 lun 0
da0: <sandisk cruzer="" fit="" 1.26="">Removable Direct Access SCSI-5 device
da0: 40.000MB/s transfers
da0: 7633MB (15633408 512 byte sectors: 255H 63S/T 973C)
GEOM: da0s1: geometry does not match label (16h,63s != 255h,63s).
GEOM: da0s2: geometry does not match label (16h,63s != 255h,63s).
?

List of GEOM managed disk devices:
  ufs/pfsense1 ufs/pfsense0 ufsid/4ee6a8ade180251f ufs/cf ufsid/4ee6b1ad1fcce77b da0s2a da0s1a da0s3 da0s2 da0s1 da0

[…]

mountroot> ufs:/dev/ufs/pfsense0
Trying to mount root from ufs:/dev/ufs/pfsense0
WARNING: / was not properly dismounted
Invalid time in real time clock.
Check and reset the date immediately!
Configuring crash dumps…
Mounting filesystems...</sandisk></sandisk></sandisk> 

pfSense-memstick-serial-2.0.1-RELEASE-i386.img (bootlog)

uhub7: 3 ports with 3 removable, self powered
ugen7.2: <sandisk>at usbus7
umass0: <sandisk 0="" 2="" cruzer="" fit,="" class="" 0,="" rev="" 2.00="" 1.26,="" addr="">on usbus7
umass0:  SCSI over Bulk-Only; quirks = 0x0000
umass0:0:0:-1: Attached to scbus0
da0 at umass-sim0 bus 0 scbus0 target 0 lun 0
da0: <sandisk cruzer="" fit="" 1.26="">Removable Direct Access SCSI-5 device
da0: 40.000MB/s transfers
da0: 7633MB (15633408 512 byte sectors: 255H 63S/T 973C)
SMP: AP CPU #1 Launched!
GEOM: da0: geometry does not match label (16h,63s != 255h,63s).
GEOM: da0: media size does not match label.
Trying to mount root from ufs:/dev/ufs/pfSense
Invalid time in real time clock.
Check and reset the date immediately!
Configuring crash dumps…
Mounting filesystems...</sandisk></sandisk></sandisk> 

Any help is appreciated.