I am about to embark on a project to re-develop a Windows Mobile 6 application; I originally had imagined doing it for Android, but the powers that be have decided they wanted a cross-platform solution.
As such, I've been looking into various cross platform solutions, and it came down to a decision between monodroid/monotouch or Rhomobile.
I am not really happy with either choice from the little I've played with them. I've found very few accounts (basically none) of people using Rhomobile and problems they faced.
The application is very data-centric, and it is essentially all about data entry. However, the data it handles is complex and there is a lot of it. The application will potentially have to run for long periods of time, up to a week, without being connected to the internet, so a Web App optimised for mobile is not a solution. Rho concerns me as I can't find any example beyond the trivial ones, and I've struggled to do some reasonably trivial things as well (though that maybe down to my lack of experience with Ruby & RoR).
Most of my experience is with ASP.net (web forms rather than MVC unfortunately). The app would be distributed internally; it's not going to end up on the general marketplace for Android or Apple. Android & iPhone/iPad are the most likely platforms though Windows Phone maybe used later.