If you use on the apps that preloads map data then cell/data coverage is no longer an issue. I find my phone harder to see on the bike than a gps though. Plus you have to modify your glove (or take it off) to use the phone touch screen on most phones. Lastly, I notice my phone gets warm in the sun on my 1200GSA. Part is probably air flow from the oil cooler too, but some phones with shutdown if too hot.
So, I'd say yes, you could do it. Play around with some of the free apps somewhere where you have data plan coverage first and see if you like it. I think Waze is on both Andriod and iOS and is free. They have added (or are adding) routing. On Android just use the google maps, which now will cache map data so if you create the route and preview it before you head out then it will already be on the phone and won't need a data connection.
Garmin also has apps for both platforms, but I haven't tried them. Too cheap to spend money on nav apps since I have a couple of working gps's. That being said, I guess I have bought Motion X drive for my ipad.