It's interesting to see what Vodafone are doing in Germany.
http://www.theregister.co.uk/2005/07/18/voda_vs_voip/
5Gb of 3G data usage for €34 per month providing you stay in your home zone and 1000 mins of local & national calls for €20 per month again if you stay in your home zone. Combine this with one of the 3G voice/ethernet data routers that allow you to plug your PC's and existing phones straight in and you have a rather interesting setup.