Here’s a quickie …
Today, Nielsen released stats for the the mobile phone market. From November of 2010 to January of 2011, Android represents 29% of market share with Apple and Blackberry each at 27% . However, without time constraint, total market share for Android compared to all smartphones is at 19%. That’s fairly rapid adoption.
Blatant opionion: I don’t think any iPhone users are jumping ship to go to Android. Former Blackberry users and first-time smartphone buyers are snapping up Androids (probably due to cheap price … and the fact that you aren’t forced into a draconian contract with AT&T’s sub-par service). Windows Mobile will always have its little sliver of the market.
Blatant opinion 2: If you’re considering building an iPhone app, you need to consider an Android version as well.