I've been trying to figure out how to build a Flex 4 based iPad application for a few days now. Getting a plain old AS3 air project was trivial, but there wasn't a lot of info around to get flex working, and it was giving me big horrid errors.
Step 1: Sign up for Apples iOS developer program. Yes, you have to pay the $99
Step 2: Go through their process to get a signing certificate and a provisioning profile. At some point during that wizard you will have to enter in the UUID of your iPad you want to develop on. After this step you should have a signing key (from apple, not your AIR one!) in your keychain, and a .mobileprovision file from apple. Step 3: Export your signing key from your keychain app (on OSX) to a .p12 file on your filesystem. Give it a password when you do this (I tried passwordless and it didn't work for me) Step 3: Install the iOS SDK from apple - big download, might as well start this ahead of time. Step 4: Download the iPhone packager from Adobe - http://labs.adobe.com/technologies/packagerforiphone/