I use a Garmin Nuvi 765T for that. It's one of three GPSs (755T, 765T, 775T, 785T) that Garmin made that can play MP3's, far as I know. Actually I just checked their site and they make two motorcycle units with MP3 players, but they're ridiculously expensive.
It's pretty nice; it even pauses the music while it's giving spoken directions. My only complaint about it is the touchscreen isn't the most responsive. It's been discontinued for a couple years now, but I'm sure you could find one on eBay if you really wanted.
I like the idea of doing a smartphone setup though. Seems like you'd have a lot more capability than a regular standalone GPS unit.