Skype is pushing itself into new territory, following an announcement today that their Skype Lite client will soon be making its debut on Android based phones, along with other Java-supporting phones. The application has been submitted today, and will hopefully be available for download within the next few days. Skype Lite will be native Java, and as such will run on a variety of phones. Couple that with Wi-Fi support, and you could in theory have a phone that doesn't even need to rely on cell towers to make a call – assuming you can find a wireless network.
There are other advantages to having a VoIP client on a cell phone, such as the lower international rates, along with the management functionality that VoIP clients often offer. Cell providers often charge very high premiums for international calls, as well as dinging people pretty severely if they exceed their plan's monthly limits. A VoIP service coupled with a cell phone could be very useful to many people.