VOIP on top of cellular?

a href=”www.outlook4mobility.com “>Andy Seybold, in his latest email newsletter, calls push-to-talk the "killer app" for wireless voice networks. Only Nextel has it today, becuase its network is based on two-way radio technology. Seeing Nextel’s numbers, all the other carriers are developing push-to-talk. What’s interesting is the way they will get there: "There are many companies including FastMobile (fastmobile.com), Mobile Tornado (mobiletornado.com), Winphoria (winphoria.com) and Somin (sonimtech.com) in the business of enabling push-to-talk services over digital wireless voice-and-data networks. Most of these players are using voice over IP (VoIP) to provide push-to-talk services rather changing existing digital voice services." In other words VOIP could be a big technology over mobile networks, alongside their existing voice services. Who woulda thunk it?