There is a firmaware restoration script mentioned in the forums but it is very hard to find.
So if you want to upload a new firmware and cannot use the web interface (either because the uploaded firmware is incompatible with the running one or because you do not have a web interface anymore)
1) unplug the wl500b/g, press and hold the restore button, plug the power in, release the botton. Power led should be fashing.
2) connect one of the lan ports on the router into your computer. (the corresponding led should light up)
3) set your ip to 192.168.1.x, where x is from 2 to 254
4) run either the firmware restoration tool from the utilitis shipped on the asus cd or use recover.sh for GNU/linux, unix,.. http://jaffar.cs.msu.su/wl500g/recover.sh http://wl500g.info/showthread.php?t=134
Thanks oleg for writing the script
5) the restoration may last a few tens of seconds after the upload finishes. Maybe the led may stop blinking when the firmware is flashed or something.
6) unplug wl500 and plug the power back, the new firmware should be booting. If the power led is flashing you probably did not flash the firmware correctly or it was corrupt. Try again with a different firmware.
Any additions and corrections welcome