http://www.wired.com/2015/03/googles-new-wireless-service-will-change-internet/

Sounds like they'd initially be modeled after Cricket, Republic, Ting, etc where they piggy back on existing networks, most likely Sprint and/or T-Mobile.  Though if it goes well, they'd probably end up buying one of those two.  

More competition is always a win for the consumer, but I do worry that Google is getting too big.