It seems like femtocells are are a half way solution. Leveraging a dual mode smartphone would provide much easier solution. Maybe a phone based client that automatically routes through the WiFi to the cellular infrastructure would be a better solution.

I know t-mobile has a solution but that still requires you to get a separate router and it is limited to certain phones. .

I've looked at enterprise solutions like Agito Networks, but the cost seems prohibitively high for non enterprise users..