If you download the api you can go to a 'mobile' folder (arcgis_api_for_flex_3_0\ArcGIS_Flex\mobile...) and load in 2 flex projects into flash builder. They are not the flexviewer equivalent to mobile, but they show you a starting point. Also, if using the flash builder 4.5 or 4.6 I think you can package up your mobile project for apple/android/blackberry all from one code base! I was just curious once so I loaded these sample projects into flash builder and emulated them on a couple different mobile devices, but that's as far as I got.