WiFi Cell Phone

After playing with Asterisk and the extremely easy to use Asterisk @ Home I'm now thinking of all the wonderful possibilities of VoIP. WiFi cellular hybrid phones are now coming to market and it would be wonderful to piggyback on open WiFi networks instead of cellular network. Unfortunately I see two problems. 1. You can't pass a call between a cellular carrier and your internal PBX. 2. Firewalls. Unless it's your own WiFi set-up or a trusted source chances are it's not going to let a SIP connection through.