Flex 4.1 includes Flash Player 10.1 and AIR 2.0
If you want to start developing against the new runtimes that where released recently in Flash Builder 4 you’ll want to go grab Flex 4.1. Once you have pulled down the Flex 4.1 SDK and unzipped it you can point your Flash Builder projects towards the new location. To see the new API’s show up in code hinting you’ll need to change the target Flash Player version in the project’s compiler properties section to point to 10.1.0. See the picture below:
If you don’t point the project settings to 10.1.0 you will not see the new Flash Player 10.1 API’s, like MultiTouch, Geolocation, and Accelerometer to name a few. This is needed for Flex browser projects but for AIR (or Desktop projects) it will pick up AIR 2.0 by default and pull in the newer API’s. You’ll see a Flash Builder 4 update at some point that updates all the tools with the latest runtimes, for the short term this will get you started in the right direction.