DingoTel has a device called the DingoTel 2Way that lets you use any 2way radio and do push to talk over the internet. The service is like Nextel but it is free. There are no monthly charges because the service runs on a peer to peer network. Check out http://www.dingotel.com/2way/index.asp
They also make a phone call service that runs on standard SIP but they have "fixed" sip so that you don't have to put in those annyoing firewall port mappings... This makes SIP work behind firewalls and restricted corporate networks. check out http://www.dingotel.com/Pc/sipt.asp
These guys are coming at it with a very different approach... lets see if they can bring in a new realm to the boring old topic of VoIP, Skype and Vonage.