As a data point, I'm using LimeWire 4.9.28 Pro with Java 1.5.0_04 on a LAN connected to a NETGEAR WGT624. When I first brought up LW, it said I had a firewall between me & the 'Net, and nothing I changed in the config would give me pretty bars.

I exited LW and was about to admit defeat, but decided to bring it up again. Within a minute, the firewall brick wall icon went away and I got five blue bars with a Turbo-Charged connection. Using the http://192.168.0.1 admin interface to my router, I see that there are two published forwarding ports (6346/udp and 6346/tcp).

Apparently, with some routers and some versions of LW, the magical UPnP stuff does work.