A lot of people have been asking whether the Adobe AIR beta currently works on Apple Leopard (OS X 10.5). I pinged the team and they they told me that they are currently doing some testing, and havent run into any major issues yet. However, we are seeing some reports of issues from developers online, so if you are currently working on an AIR app, you might want to wait a little while to upgrade until we have more information.
I installed AIR on Leopard, and all of my apps ran fine (although there were some minor issues with badge install).
I don't have any information on Flex Builder, but I will try and install it and see if it works for me.