DD-WRT cannot set the speed/duplex on the Linksys WRT54GL's switch. You can set it up, it'll show up in the interface, but the hardware will still run at 100mbps/full duplex. It may work on other WRT54G models, but not the GL. DD-WRT micro uses too much flash, so there's not enough space to install ethtool. I ended up using OpenWRT, installing ethtool, and setting port speeds at command line.