A quick way to setup a MOBI site is simply to install wordpress and then use the WPTouch plugin. I have tested it on a couple of iphone's and htc android and it works excellent. Not sure about older mobile browsers though. The plugin company say it works with iPhone, iPod touch, Android, Opera Mini, Palm Pre, Samsung touch and BlackBerry Storm/Torch mobile devices. It auto detects the phones so it shows the mobile site for those and shows the normal wordpress site for any other traffic.