I\'m currently trying to decide wether to accept a client\'s proposal or not. Basically, I\'m asked to create a MapView that displays markers at several locations on a map,
Have you looked at alternate map frameworks on the iPhone? I know there is at least one open source map engine, also with tiles (that are not as good as the Google tiles, but hey).
A decent set of them is here:
Creating an IPhone Map application
The "easiest" way to do this within the Google Map framework is simply to map the client's map as a texture on top of the "ground." You can create textures at different resolutions, for different zoom factors. Then you won't need to do any special coding at all --- everything will just work.
The way you do this is with a KML region that maps to ground level.
See: http://earth.google.com/outreach/tutorial_region.html
You can't use custom tiles with MapKit. You're limited to using the ones provided by Google.
It could be easier to create a "Google Maps-ish" web app that uses the custom titles and can be viewed on the iPhone through UIWebView?