For multi-platform apps you could go with either PhoneGap or Xamarin. With Phonegap, you will be working with Javascript, HTML and CSS to construct the UI of your mobile app, whereas with Xamarin (previously known as MonoTouch) you'll be working with C#.
Xamarin I have not used, but my experience with Phonegap for Android was not that good. The UI components were not as responsive as the components in native Android apps. Everything worked, but there was still this slight lag that you definitely noticed when e.g. clicking buttons, selecting items from dropdowns and so on. It was fine for testing and demoing, but I would not have distributed any Phonegap-based apps to actual end users.
This was a bit less than a year ago, though, and Phonegap has had many updates since then, so the situation may have improved. I have to say that using the Phonegap API to control the camera etc. and being able to sketch the UI with HTML and CSS was great already then.
Another thing to consider is that do you need the app to look and feel the same way as a native app would? If you do, making PhoneGap (and I expect Xamarin too) mimic the native UI will take a lot of work.
From OpenERP's point of view it makes little difference what language/platform you pick, it mostly boils down to what languages you're already familiar with and to what targets you end up needing to deploy on.
If you're not completely sure about the need for a multi-platform app, you could get started with Android first and do a proof-of-concept type of app with it. It's quite easy to get started with it, and there are tons of Java/Android tutorials available.
