Having done much of my new flash game screens in 640x480 4:3 ratio, I have noted on some of the major game sites for submissions a 16:9 is desirable, but not mandatory. Because of my image work I would bump to 800x600, keeping that 4:3 ratio. Changing will be an undertaking.
Some of the recent games here in the forum (Going Bananas) are at that ratio. Has ANYONE had any issues with flash game submissions using the 4:3 ratio recently?