mobile/cell phones with sip and wifi are a great choice if you have a mobile contract anyway. i use the nokia n80i (n80 flashed to internet edition to give sip functionality) so far testing has proved it to be pretty good for sip calls, only downside so far is batterylife if leaving it connected all day. the nokia eseries phones are the main ones to look at. i think the newer ones (inc n95?) are going to have improved firewall/nat abilities -but Im not sure on that.