I have recently been working on a mobility project for my company.
Using a Windows Mobile phone (PDA), I can access any Wireless Network (work, home or public hotspot) then connect through a secure VPN tunnel to our Corporate LAN.
This is incredibly cheap - Public WIFI hotspots (BT Openzone & affiliates in the UK) cost 1p per minute!
In fact, it is so cheap that our Telco division wants to use the WIFI connection to run VOIP to our corporate voice network as it works out CHEAPER than making a mobile phone call!
So instead of using a mobile phone to support data transfer over a voice network, we may soon be using the mobile phone as a data device to run voice calls. This is going to confuse the hell out of the poor end-users!
Of course, the device has a GPRS-enabled SIM card, so outside WIFI coverage it is possible to make ordinary mobile calls and use GPRS for data - but as others have pointed out, this is sickeningly expensive.
The reason that GPRS / 3G is so expensive is because of the vast sums that were spent acquiring the licences in the great Government auction a few years ago.