SKype works fine on 2G, 3G as well as Wifi !!
But one issue which i found while using on Wifi was that , the person using Skype through Wifi (on some other phone) makes skype call, we can hear him, but he could not hear my voice. In that case, i had to call him back from skype (using my 2G/3G plan) and then we were able to communicate properly.

Had experienced this behaviour 2-3 times