Mobiboo have recently launched in the UK with the first wi-fi mobile phone network. The network is being provided in partnership with theCloud and they currently offer three tiers of account, ranging from Free Trial (which includes a 07911 number, softphone and 1UKP credit) to the Plus + account which includes a Starcom L1000 wi-fi handset, free voicemail, 10ukp credit, 07911 number, free voicemail and "follow-me".
I think this is spot-on for the die-hard wi-fi starbucks addicted techno-freak, but this segment will always need a GSM mobile in their back pocket for moments when coverage is poor or unreliable. However, couple this with an MVNO deal, reliable network hand-over, and dual-mode handsets and you have reliable nationwide coverage. The tricky bit is going to be making this digestable to the mass-market consumer - ensuring the technology is hidden from the service experience so that they're not hopping for signal on one foot and balancing different call costs on the other.