I agree. AS3 actually has the most incredible drag and drop support and it is also very easy to implement via Flex. In fact, all List based components have this functionality native to them.
But adding it to a non list base component isn't awfully hard either. Look in the docs for drag and drop and look for "Image to a Canvas" examples on drag/drop - I believe that ought to answer many of your questions. Good luck with the app. Sounds heavy. -Kenny
