Check this out: the Touchmods team have released version 1.0 of SIP-VoIP for the iPod Touch which will allow the Touch to make phone calls over its WiFi connection. Watch the demonstration in this video:
While Apple is seperating iPod Touch and iPhone into two different product lines, the release of such hack is starting to blur out the line. People who hate to sign 2 years contract with the phone company (which is required for iPhone) can now choose to buy an iPod Touch.
There's a limitation though: phone call can be made only when you are in WiFi coverage area..