Yes, I can (and have) set up a VPN connection using a wifi PDA and made calls using a softphone client - but it takes far longer than just using the damn thing as a mobile phone! Plus - it only works for outbound calls.
Skype will sell you an 0208 (or any other national) phone number of incoming calls, but then the receiving device (PDA or laptop) has to be switched on all the time, and the battery technology just isn't there, not by a very long way, to keep a pocket/pc or windoze XP device awake for days.
It's also possible to use 3G in the same way - but the 3G usage rate costs more than just making the call over the mobile. Plus the data rate is barely adequate for voice.
I did wonder about this. Presumably the data is priced to prevent VOIP being cheaper than GSM?
The lousy 3G coverage is another thing. The M25 may be covered but what about France, Spain, Greece, Croatia, etc etc...
On some of the juicy business contracts, say £50/month, you can get say 1GB "free" data. One could do a lot of VOIP within that allowance. Unfortunately the allowance disappears once you go abroad, although there are again special business deals for that.