Brainslayer has released DD-WRT v24 pre-RC4 and it supports another two of D-Link’s router, namely – DIR-300 and DIR-320.
He’s kind to enclose this flashing guide. He flash his DIR-300 using console cable while I did mine without one. After skimming through the guide, I would say the process is similar to flash the fonera (which is also based on Atheros chipset + RedBoot). The tricky part would be breaking out of the boot script under 1 second. Nonetheless, I would say the flashing process is pretty safe (after I have flashed so many foneras and routers through RedBoot/Broadcom console)
My DIR-300 which is running DD-WRT now.
The usual tools needed would be a tftp server, a telnet client which allows you to send Ctrl+C (hyperterminal on windows would work fine), an ethernet cable and the DIR-300 itself.
Check out the flashing guide