Someone mentioned Titanium earlier. Titanium shares the major advantages Paulo points out in his case for Xamarin (i.e. single language, cross-platform, native apps, access to native APIS). But Titanium also has a couple clear advantages over Xamarin:

  - Titanium is FOSS - Xamarin's cost is merely very inconvenient; its closedness is much more repulsive
  - Titanium doesn't sideline those of us who develop on linux (a linux-based OS rules the mobile market, after all)

On the other hand, C# is a beautiful, evolved language that I would love to try out with mobile development, and I can appreciate a shiny IDE. However, as long as the above conditions apply, I'll always consider open and developer-friendly solutions like Titanium first before resorting to Xamarin.