Is it possible to get an AIR 3.0 Captive Runtime app into the Mac App Store? How would you do that?
There are actually a couple of steps to doing this. First, and this is what I have found to be the easiest solution, is to build an AIR Intermediate file from Flash Builder 4.5. Once that file is built, use the adt command line packager with the "-target bundle" flag to sign and package the a Mac OSX application bundle. This will result in a fully working captive runtime application bundle that you can run on OSX. When you have completed the Adobe side of things and have verified that is runs correctly you can move on to the Apple side.
I would recommend you reference Apple's developer documentation for this here: Submit Your Application using Application Loader. You will need to use both the "codesign" and "productbuild" commands on the application bundle you created from Adobe's packager. When done, you should now be able to use Apple's Application Loader to submit the package to the Mac App Store.
I have not yet tried to sign the AIRI package with my Apple certificate, so I'm not sure if that would work, since I have both an Thawte cert for Air apps and the Apple issued one. This would take further testing.
Here's a link where you can find pretty thorough article how to upload an AIR app in the Mac App-store.