I just recently decided to experiment with Mono for Android (www.xamarin.com). I had no problems installing it on my Mac Mini running Lion but I had no luck with my Windows 7 laptop.
The installation would appear to complete successfully, and I would see the SDK from within MonoDevelop but as soon as I'd try to create a project I would get the pop-up stating "Mono For Android is not Installed."
Additionally, every time I ran the Mono for Android installer it would tell me it wasn't install (but all other prereqs installed correctly).
After several re-installs I noticed devenv.exe for Visual Studio 2011 Beta was constantly running.
So I'll cut to the end, I needed to fully uninstall Visual Studio 2011 Beta, uninstall Mono for Android, and then re-install.
Now everything works great.
For reference I'm running Windows 7 Professional 64, I have Visual Studio 2010 Ultimate installed (thank you Bizspark) and I installed Mono for Android 4.2.2.