I have done a bit of searching and can only fins some old posts about some Linksys hardware. Obviously, M0n0wall can not run on the older Linksys stuff, but Linksys uses different processors in their hardware and they come with different amounts of flash mem and ram.
WRT160NL is specifically made to take advantage of open source firmware. I runs on an Atheros AR9130 rev 2 (0xb8) (400 mHz) and has 8MB flash mem and 32MB RAM. You can see some other stuff about the processor here:
http://wiki.freebsd.org/AtherosMIPSProviding FreeBSD is able to run on it, I would absolutely LOVE to see a version of M0n0wall running on Linksys WRT160NL. It would be a fantastic firewall and router for the home and SMB market, especially with the IPv6 features.
I am running DD-WRT today, but I have not seen any news about new versions or development since I installed it. It is stable, but all IPv6 stuff has to be done from CLI. Also, I have a little more faith in FreeBSD on the network side :-)