This can already be done on Symbian phones using 'JoikuSpot' and windows mobile using 'WalkingHotSpot'
Without even using those software packages you can dial into the 3G network /GPRS without any software on both Windows and OSX using either a cable or bluetooth.
Personally using an N95, Bluetooth and OSX on Vodaphone UK I can get 60-90 KB/s download speed on the when on 3G, 5-8 KB/s when on GPRS.
A bluetooth 2.0 dongle is £4. Internet can be shared over ethernet and WiFi easily on both OSX and Windows.