I agree pretty much with IP540 on this one.
I'm running a project for a corporate client at the moment where VoIP softphone software (and hand / headsets) is being distributed to laptops, so that the corporate users can connect to the the Corporate LAN over a VPN and then use the softphone to plug straight into the Corporate PBX. Instant free internal calls plus least-cost routing and local breakout. Great savings, everyone happy.
BUT it is only practical to use this from home / client site, where one is going to be sitting in the same place most of the time.
For everything else - the mobile phone is just so much quicker and easier - even then a wifi PDA, just as IO says above.
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.
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.
Maybe when HSPDA arrives and usage costs are driven down it may be feasible - but then, the distinction between a voice call over a digital mobile cellular network and a VoIP call over a 3G cellular network becomes pretty blurred!
SD