I'm using Debian with a 2.4.12 kernel. I have a utility called FWBuilder installed, to allow me to manipulate the iptables stuff graphically. My laptop (Win 2000 most times :-) ) sits behind my linux firewall, and I use Bearshare and/or Phex with no problems. You just have to configure iptables to forward whatever port you are sharing on (default 6346) to your internal machine (in my case the laptop). Everything works fine, I just have to configure my Gnutella client to specify the IP address as the one on my external interface.

The actual iptables lines that configures the forwarding is

#
# Rule #0
#
# Gnutella
#
iptables -N RULE_0
iptables -A OUTPUT -p tcp -m state --state NEW -d 192.168.0.2 --destination-port 6346 -j RULE_0
iptables -A FORWARD -p tcp -m state --state NEW -d 192.168.0.2 --destination-port 6346 -j RULE_0
iptables -A RULE_0 -m state --state NEW -j LOG --log-level debug --log-prefix "RULE 0 -- Accept "
iptables -A RULE_0 -m state --state NEW -j ACCEPT

but I highly recommend fwbuilder.

HTH,

Gollo.