Make sure the flash plugin is enabled and go to http://www.dominos.com.au/home/ and then click on the huge "order online" button. (you don't have to be in oz to reproduce the issue). Now with the two flash movies playing, look at Safari's CPU usage in top. On my computer box, it is around 79%. A similar problem happens with the ads on http://www.nytimes.com/2007/08/02/us/02bridge.html?_r=1&hp=&adxnnl=1&oref=slogin&adxnnlx=1186027285-8Uu/CbTQXNhjogdbpnioag (if you get flash ads). This becomes a significant issue when you're on a laptop, in the shower, running off of battery and a Dell ad is looping in the background. The battery quickly drains. This may become a very loud issue in the future if people start complaining their battery doesn't last 2 hours (when they actually have an unknown flash ad in the background). Now, it'd be nice if someone could get Apple's evangelist people to bug Adobe to fix the problem in flash itself, but I don't think Adobe would fix it based on another bugs/implementation details that exist in the flash plugin.
For what it's worth, I think this high CPU usage happened even with Safari 2.0 which is prior to the removal of the plugin event limiting code.
On my G4, I'm getting ~60% CPU usage from shipping Safari/WebKit, and ~80% from a nightly with Safari 3 beta. Given that there's video playing, I'm not sure if this is a particularly bad result.
Oh, I should mention I'm using the beta flash 9 quartz plugin, which is significantly "better" http://labs.adobe.com/technologies/flashplayer9/
I should note that the video on a background page becomes jumpy for me even with Safari 3 when playing another one in front window, probably because the processor isn't fast enough. This explains why the difference is not so large.
NPAPI support is gone and this website was primarily using Flash in back old days. I think this can be marked as "RESOLVED WONTFIX". Thanks!