Thanks for your reply.
I may be failing to understand the API docs, but the docs do suggest that PictureFillSymbol has a default and if I remove the source property from my code a blue push pin does infact display (in earlier versions of flash player).
I've checked and the polygon is displaying correctly if I remove the PictureFillSymbol altogether.
The image I am trying to use is embedded, so I dont think it can be a security sandbox problem accessing the image?
I should have been more clear. The PictureFillSymbols appear perfectly in Firefox with Flash Player 10, but the same map does not display the symbols in IE with Flash Player 11. Instead I get a bunch of vertical coloured lines as I pan and zoom the map.