Yes, there is a workaround. A loophole, if you will. It involves tethering your iPhone to any mobile hotspot (which is in turn connected to a 3G cell network). From the guy who put it together:
I’m just putting two and two together here, so hear me out: if an iPhone 4 can only make FaceTime calls over WIFI like the iPad 2, could an iPad 2 make a FaceTime call over 3G by connecting to an iPhone 4’s Personal Hotspot feature which is WIFI?
Pretty cool, eh? It’s not quite perfect since it requires a hotspot, but if you have one, you can Facetime from anywhere a 3G network is available.