From the dd-wrt.com homepage:
"04.04.06 2:32 am
Subject: and last but not least... ...i added now a 1,7 mb micro build to the dd-wrt series and yes it runs on the WRT54Gv5 too if you know how to flash it. so please keep the donations running to support my work in future too. "
Look at the betas dated April 3, 2006 or later. The "final" v23 SP1 should be coming soon.
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Needless to say, due to less available memory, there are fewer features than would be available for other routers. Since it was just released into beta, there isn't alot of information yet. You can keep an eye on dd-wrt's fora and the DD-WRT WIKI.