Very interesting. Do BT have something like Vodafone's SureSignal in a USB form to connect their routers to customers local mobile phones.
FON doesn't really work well in my opinion but if the range was good on these add ons and BT was to give them out free or dead cheap they could build a pretty big (although patchy) 4G network pretty fast. Combine that with a spectrum sharing agreement with another 4G supplier and there are some interesting concepts to explore for all parties involved!
BT - the next OTT provider thanks to 4G - The Full Spectrum