I contacted Adobe support and told them about the problem, and their response was basically, "it's not our problem, other stuff is also hogging lots of CPU in Snow Leopard too", referencing the post elsewhere on here about the Spotlight indexing service hogging CPU. Also, they said that Flash is not officially supported on Snow Leopard yet.
Anyway, I found a fix for this problem, at least temporarily. Install the
ClickToFlash plugin. This disables all flash movies until you click on them. It seems the reason for the excessively high CPU isn't watching 1 flash movie, but having many flash instances on one page seems to nuke it: now when I run a Flash video the player plugin only uses around 40% CPU, which is at least better than 150%. I'm guessing that ad banners and so on on the same page stack up and with each instance using 40% itself it pushes things too far.
Also has the nice side effect of effectively blocking those annoying ads that take over the whole page when you accidentally mouse over them 🙂 Anyway, give it a shot and see how you get on.