You can just open Safari on the iPhone or iTouch and load in a standard Mendix application.
It will work, but the load times on networks like EDGE are lousy and the UI of a standard Mendix app just doesn't lend itself well to the iPhone. Network load and UI issues aside, Safari on the iPhone seems to perform rather well (for an 800MHz CPU).
A web-based 3G iPhone UI based on Mendix sounds plausible.
Make some noise and get people to care. The web-based Mendix client also runs on the OLPC and the Wiii, but they're not real development targets either. They are better targets than IE6, though, and Mendix can run there as well...
On the iPhone home screen click Safari, then type the url of the Mendix application, and it works fine immediately!
What about using in Blackberry ?