Its been a while since I looked into the gaming side of the Flash. In the early days it were all hacks to get things done for an effect to happen. Now as the Flash platform is matured and put itself as a major player for game development, the complexity of the development also increased. Fortunately all the good things, which happened to the matured languages, is happening to Flash now. There are some nice frameworks and engines coming in. When game programming demands a completely different mindset and process, the conventional application development frameworks can not be fit in this place. People and companies have tried and ported the best game engines and physics engines to flash. Below is a list of 3 frameworks/engines , which are kind of setting themselves as the major gaming engines in Flash.
All of the above engines are opensource and already showing a fan following and promise to deliver quality gaming.
Personally I have not yet tested anyone of these. Will update once do some hands on.