edit: Rather than do this, you might now be better off getting Ricardo's updated project from github, switching to the adt-20
branch, building with Maven, uninstalling the M2E connector as described below, and installing the new M2E connector from me.gladwell.eclipse.m2e.android.update/target/repository as a local repository, also as described below. Thanks, Ricardo!
I had this problem and couldn't build my project. The only way I got round it was to downgrade ADT to 18.0.0. I couldn't find a way of doing this through the SDK Manager etc., so this turned out to be a bit of an ordeal - though I quite probably didn't do it the most sensible way! Anyway, I thought I'd post what I went through in case it helps anyone. I left out the umpteen Eclipse restarts. :)
This was on Eclipse 3.7 on Linux.
- Get the ADT zip: http://dl.google.com/android/ADT-18.0.0.zip
- Get the Android SDK Tools archive for your OS: http://dl.google.com/android/android-sdk_r18-linux.tgz (replace linux.tgz with windows.zip or macosx.zip - sorry, Stack Overflow only allows me two links in my post!).
- Disable automatic software updates: Window -> Preferences -> Install/Update -> Automatic Updates, uncheck 'Automatically find new updates and notify me'.
- Uninstall Android SDK Tools: Window -> Android SDK Manager, check Android SDK Tools and click 'Delete 1 package'.
- Uninstall ADT: Help -> Install New Software -> What is already installed? -> Installed Software, select all the Android* components and the Android Configurator for M2E and click 'Uninstall'.
- Extract ADT-18.0.0.zip and install: Help -> Install New Software -> Add -> Local and browse to where you extracted it. Give the local repository a name and click OK, then select it from the 'Work with' drop-down and install the available software from it.
- Extract the tools directory from the android-sdk_r18 archive into your android-sdks directory.
- Install Android M2E connector: Preferences -> Maven -> Discovery, click 'Open Catalog' then select the connector. Deselect any ADT 20.0 features if prompted.