If you have not heard, AIR 2.0 is out on Adobe Labs and has all sorts of new and wonderful features.
There are plenty of instructions for getting the SDK running available. But the one bit that caused me the most trouble in upgrading and using AIR 2.0 was this:
REMEMBER TO UPDATE YOUR APPLICATION DESCRIPTOR NAMESPACE
In the xml file that accompanies your air application do not forget to change the namespace to reference 2.0beta.
If you forget to change this, your application will compile just fine and you will not see any errors. But when you run it there will be errors about classes not existing and all sorts of other junk.
So please remember when upgrading to AIR 2.0 to update your application descriptor files namespace.