I thought about this myself. A couple of things:

If you intend to strap the device on the handlebar where it's easily visible, you'll need something waterproof (or water resistant, if you settle for that). Not many such GPS enabled phones out there. There are ways around this of course, but you may or may not be happy with the device in a clear plastic bag/cover or inside handlebar bag.

It would be a good idea if the device was rugged too. Even if you didn't do off roading, it will get its share of vibration and the occasional crashes. As far as mobile phones go, ruggedness and waterproofing tend to come in same package - at the cost of screen size. IMO, an iPhone-sized color screen is about the minimum for mapping GPS. If you settle for numerical coordinates only, you can get by even with a largish wristwatch GPS unit.

Battery life. Many of the features required for this package are power hungry. GPS, GSM, digital compass (if any), relatively large color screen for example. If you can switch off functions when they're not needed, that would be good. Odds are, you can only switch off the GPS and compass feature. I haven't found much comparable data on battery life with various options, but maybe users can share their experiences. I doubt one can squeeze out a full touring day's worth of use, with GPS constantly on while riding.

Map costs.Some GPS phones download maps on the fly as needed. Even if the map service itself didn't cost anything, data transfer will, depending on your provider package. Data transfer costs can be really high when roaming outside of your local cell phone network.

As you can probably tell, I chose separate devices, but I also needed them for other purposes than purely biking/touring. Most offerings failed most noticeably in ruggedness and waterproofing, and like I said, no easily comparable data was available on battery life.

--J