As you say, that's great if you're in a location where you have constant wifi access, but I think we are way off the kind of coverage required to make it useful. Don't get me wrong, I totally think the idea is great, but think it will be to a restricted number of users for a long while. If you only travel around the location where you have constant access, great. Simiarly, it would be cool to stream audio from your home MP3 collection, view TV over the net sourced from a location whereby the receiver is fixed or play movies from your HDTV.
I suppose an extension to your idea would be that when you wish to make a call, the front end searches for a wifi connection and makes a VOIP call if one exists. If none exists, it then makes the call via the mobile phone. So the cheapest option is automatically sought transparently.