Investigated how you get extra software onto a a Linksys wrt54g once it has modified firmware installed.
Batbox has a Snort-specific page but I wasn't convinced the build process was uptodate. It certainly wasnt straightforward.
The SeattleWireless page about the wrt54g is an extensive wiki but again it's hard to tell what is really current.
The OpenWRTCore approach may be closer to what I was hoping to find. It's intent is to offer a core working system with the potential to add any of a series of user-chosen packages for the funtionality you want.
Unfortunately, the OpenWRTCore site seems to have no packages.
There's information about Kismet on the Linksys. Looks like those
instructions are good and there's an associated discussion thread on the page