After upgrading Eclipse Helios to Indigo described in our article here, Android development platform should be upgraded, too. Otherwise, and error message will appear informing you that Android platform version is not valid.
How to upgrade Android platform on Eclipse
1) Window > Android SDK Manager
2) Android SDK Manger will perform checking for new updates
3) Select desired packages
4) Click Install packages...
5) Select Accept All and click Install
6) Restart Eclipse