New install of Flash CS5.5, and I just opened it up after three weeks and the latest update. I create a new AS3 document, 940 X 226 with a black background, and work on several animations. I close the document, and click File>New, and choose a new AS3 doc. Now I go to the Properties window and it's all greyed out. It says 940 X 226 with a black background again, but the document on the stage is the default 550 X 440 with a white background.
The ONLY way to open a new document that I can change properties on is to close Flash and reopen it.