Linksys have released a new Firmware for the WRT54G (US model). Changes are as follows:

Firmware Version 1.42.2

Updated wireless driver to 3.31.15
Added URL and contents filters
Added L2TP passthrough
Added HeartBeat Signal support
Resolved issue with Connect on Demand

Firmware Version 1.41.2

Updated GUI format and layout

Afaik the 1.0 and 1.1 versions are the same but still get the 1,0 version here:

WRT54G 1.0 Firmware

and 1.1 version here:

WRT54G 1.1 Firmware