So far, the devices are expected to be available in retail sometime next year. Considering Google's strong lobbying for an open 700MHz spectrum and its potential participation in the upcoming auction, they may be planning a coupling here of new spectrum along with new hardware.
Another angle is that Google might seek to infiltrate many mobile platforms via creating an open platform with which to develop and push many types of Google programs. They have already made significant advancements in “mobile” tools you can use with existing phones, so it isn't far of a stretch to think of them creating an entire platform for mobiles all their own. Hopefully they will soon shed light on what their plans are.