I believe the way I did this is to create a google maps account. Then import the overlays into google maps.
I remember it took an awful lot of fiddling as the original overlay was too big for it to process so had to split into lots of smaller overlay files. Once I had completed the task and could view them on the iPhone it seemed just fine, however it doesn't store data locally so needs a constant wireless or 3G network connection to work. So out on the trails it was absolutely useless.
