Warning!! This document describes an advanced procedure! Directly writing to the flash in this way may damage your NSLU2.
NOTE: Make sure that mtdblock5 is the one you want! On OpenWrt, you actually want to overwrite mtdblock6, although it's not writable...
NOTE2?: The above command will produce a "no space on device" error. This is normal and the process still works; I assume it is caused by dd attempting to write more zeros than the partition can hold, since we didn't tell it where to stop.
Without a valid signature, RedBoot thinks the flash is corrupted and enters Upgrade Mode. From here you can replace the flash image using the UpSlug tool.
(Thankx to Rod and Řyvind for the info)