For those of you who have recently upgraded to the latest version of the Eclipse ADT Plugin (v22), you will realise that some of your projects may be failing to build correctly, specifically those of you who use the Android Support Library.
In ADT v22, the Android Support Library is now in the Android Private Libraries section:
The problem is that ADT v22 does not automatically include the libraries in the Android Private Libraries for compilation. And this will cause your project's failure to build correctly.
To solve this, right-click on the project in Eclipse and select Properties. Select Java Build Path and click on the Order and Export tab. Check the Android Private Libraries and click OK. Clean your project and it should now work.