you can do this with Motion X but you have to convert the maps first, email them to the phone and then open them in Motion X. Google and you should be able to find details and instructions.
I tried this for awhile but I'm not sure the iphone is up to being mounted unless you have a heavy duty case. Even then I need my phone for a phone more than I need it for a map.
I kick it analog and create my maps in Google and then write down a punch list of turns (L-FM 156 to BFE...etc) and put that on my tank bag.
I found it's easier to keep the phone in my pocket and take it out when needed as long as I have cell service.
I also press the power button and screen button at the same time and take screen snapshots of maps to refer to when I'm off the grid.
I use an older jog wheel ipod for tunes so that I can pause or skip songs with my gloves on.
I'm interested to see how well the new Apple mapping works next week when I get my shiny new iphone 5.