Clients which auto-detect the IP:
- Shareaza
- Bearshare
- Gnucleus

You seem to know how NAT routers work. So do you know what port forwarding is ? You're right LimeWire is not running on the router of course, but if you forward a port to the Computer running Limewire, it will be as if Limewire was running on the Router. But to make it possible for other clients to download from this user without a push request, Limewire needs to send them the correct IP, which should be auto detected!