HTML5 looks to be a real Flash killer. Nobody will ever want Flash or SilverLight when all browsers start to support <canvas>, <audio> and <video>.
The one issue we have with lag is an occassional dropped packed that holds everything up.
Not to mention flash 10 will allow direct peer to peer connections, bypassing several hops...
Flash works fine on OSX in my experience. A good portion of our startup depends on it, and my cofounders code on Macbooks.
If only Apple were to create some hook for WebKit/Sproutcore that allowed iPhone web applications to do something with finger touches (this is difficult because of the pinch gestures require mouse/t-pad tracking - but maybe they can have a JS API hook to toggle/disable pinching for webapps)