Should I build an app that targets Android 2.2 and release it on the Android Marketplace;
Would the app be available for download and use on devices running a version of Android OS lower than the targeted version of the app? - Let's say Android OS version 1.6.
What would happen if a user (with an Android OS version 1.6 powered device) were to attempt to run the app?
Would they be prompted to update their OS or just receive an error message?
It depends what you have in the minSdkVersion field in your AndroidManifest.xml. If it is set to 4 or lower, then it will be visible to people using 1.6.
There's a good explanation here.
The android SDK is completely backward compatible using the mechanism that @Graham mentions. The other question will be "how far should you go". Use this graph to see the population of users on each platform level.
http://developer.android.com/resources/dashboard/platform-versions.html
This is pure opinion, but I would advise starting your app at API levels 7 or 8. The benefits of the APIs in EClair+ are worth losing this 0.4% still on 1.x devices.
If you are targeting 2.2, users with earlier SDK will not be able to download it.
They will see: This app is Incompatible with your device XXXXXXX
来源：https://stackoverflow.com/questions/6634330/are-android-apps-backwards-compatible