Hi, we started making one of the our largest AIR projects with Flex SDK 3.5 version, but we had some problem while scrolling for example with mouse wheel or even more serious bug like: sometimes while resizing window or changing size of some block during runtime(for example HDivideBox) , application stops respond for any user action and we were seeing a wheel of death.  I did not remember exactly why, but tried to compile application with Flex SDK 4, and some bugs are gone. So I recommend you to use Flex SDK 4 with compile option ” -compatibility-version=3.0″, you should use compatible mode with version three to compile your old projects with Flex SDK 4, as version 4 has some new architect structure and you wont wanna  fix whole code by hands to make it works with Flex SDK 4.