Hi Daniel,
It does work for me when I test using a different skin for mobile it works on my iphone (I don't have any Android device), so maybe you have it cached and need to refresh the page or something. Make sure you touch Web.config after changing user.config
I think there is more to making a mobile phone optimized experience than just changing the skin (though that will be part of the solution) and I have not yet figured out all the pieces to the puzzle to really be able to make it possible to optimize for smart phones in mojoPortal. Whenever I do get it figured out I will then document it because it would be a very valuable feature to be able to do that. It is not an easy problem, I think you can only get so far with skinning, though it may help.
If you do manage to make a skin that makes a substantially better experience for smart phones I hope you'll share it.
Best,
Joe