A great article summarising app development options can be found here if you need to get an idea of the approach that is best for you.
So the hard yards of your smartphone application design and development are complete. Now it’s time to put it in the app store, sit back, and watch the money roll in, right? Well, not so fast. At the mercy of a very public user-feedback system and with a host of complex variables to take into account, smart app developers know that thorough testing of that clever new product can mean the difference between outrageous success and painful obscurity in the marketplace... Read More here or access a PDF copy.
A snapshot of iOS versions in use in the market updated weekly - thanks David.
Latest details of Android OS distribution from Google. Android 2.3 still dominates, ICS slowly gaining.
31 May 2102: Some interesting statistics about mobile platform market shares and divergence can be seen at OpenSignalMaps .