I'm using the latest Forefront UAG with Service Pack 3, no client components enabled for testing with mobile portals. I was able to successfully brand/change appearance of UAG gateway log on pages for desktop and mobile versions.
Testing with Android 2.x and 4.x devices results in successfully pulling up the mobile gateway site with correct zoom levels/page formatting (stock, Chrome, and Dolphin browsers).
However, testing with devices running iOS 6.x (iPhone 5, iPad 2), I've found that user-agent detection isn't as stable. The first visit to gateway page results in loading the desktop version, but subsequent refreshes of page will load the mobile version. On both iPhone and iPad devices, the mobile page that loads is also zoomed out very far. This behavior is consistent with the Safari and Chrome browsers.
I did note that iOS 5.x was the newest supported version at http://technet.microsoft.com/en-us/library/dd920232.aspx. Are iOS 6.x devices simply incompatible? Or is there additional configuration specifically to get these devices to load the mobile pages at the correct zoom level?