SOLVED: I encountered this again while flashing pr1.3 and the vanilla image again. The only thing I could think of is that I hadn't rebooted the computer after shutting down xfce. Since I already knew the problem was probably HAL related, I ended up killing all instances of "hal-storage-addon" (just to be safe, you may want to kill everything with "hal-storage" in the name). Once I did that, it worked without a reboot.

Next step will be a script or something to blacklist the N900 during flashing operations, although killing the HAL storage monitoring daemons seems just as easy if not easier--it's not like we're flashing every day, right?