Android SDK for version 4.0 was just released. The setup procedure is almost same as before, so I don't want to repeat again.
But - after you install the SDK components and try to create AVD for Android 4.0, may be you will prompted with the error:
Unable to find a 'userdata.img' file for ABI armeabi to copy into the AVD folder.
It's because you have no Android 4 system image installed!
Start Android SDK Manager, expand Android 4.0, click to select install ARM EABI v7a System Image. May be you have to re-start Eclipse after new component installed.