From DD-WRT Wiki
 Linksys WRT600N
Important: Read the Peacock thread first!
As always make sure to do a proper 30-30-30 before and after updating the firmware. This is not an optional step!
 Flashing DD-WRT
Download this WRT600N mega version, which contains the special header information required by the stock Linksys firmware. Then, reset your unit to factory defaults and flash it via the stock web interface. Reset your router to factory defaults after flashing. You can then flash what ever version of DD-WRT you want to the router. Please note that some people have experienced an issue with this DD-WRT mega build when trying to flash a new version of DD-WRT using the Firefox web browser; a connection timeout error occurs. Using Internet Explorer seems to resolve the issue and allow flashing of a different version of DD-WRT.
--n0dyjeff 8 January 2011 The special WRT-600 mega version mentioned above is a little out of date and didn't work for me. Using a later version (the as-of-today recommended 14929 build) worked great. Here's a link. WRT600N mega version (build 14929). FYI, I wrote up my experiences and shared them in the "Fun with DD-WRT" resource link below.
 Reverting to Stock Firmware
Simply reset and flash the stock image via TFTP or through the web interface. Make sure to reset after flashing.
 Known Bugs
 Initial Flash May Not Work / TFTP problems
Some users may experience "Failed to Upgrade Firmware" messages when upgrading via Linksys web interface, even when using special 600N-specific builds. Also, TFTP may not work after the router has booted (Linksys fw only). The solution to both is to initiate TFTP within 3-5 seconds of power-on.