Here's a quick tip for you:
When using Eclipse project properties to change the Project Build Target, make sure the default.properties file is writeable. This may mean checking it out from source control before editing the properties. In our configuration, the Perforce file is not writable and changing the Project Build Target settings with Eclipse does not check out the default.properties file from source control. So, I have to first check it out, then modify the Eclipse settings.
Although default.properties is automatically generated by the Android Tools, the catch is it must also be checked in source control. Usually, this does not trigger an error, either. Instead, the default.properties file will just be out of sync with the settings and possible create strange behavior.