So after updating to the new ADT 17, you may experience some problems building your Android projects, with Logcat showing NoClassDefFoundError: your.main.class.
Here are the steps to fix this issue:
1. Right click your Android project, go to Build Path, and click Configure Build Path
2. Under Libraries, remove everything except Android [number] and Android Dependencies.
3. Next, under Order and Export, make sure everything is checked. Click OK when you're done.
4. The new ADT will recognize a libs folder and import everything automagically. If you have it spelt any other way, rename it to libs. This folder should contain the armeabi and armeabi-v7a folders, gdx-backend-android.jar, gdx.jar, and slick-ae.jar.
5. Next, go to the Project menu at the top of Eclipse, and click Clean, and clean all your project folders.
Compiling should be ok now.