I use Safari 5.0.3 - i know flash sucks and i'm just using youtube as an example - my laptop seems to run hotter in general now.
I'm just surprised because I don't see the same problem on either my iMac or MacBook Pro - neither of them are speed demons; my MacBook Pro has video decode offloading (late 2009 with a 9400M video card, 2.53Ghz processor, 13.3inch) and my iMac uses CPU decoding - neither of them exhibit CPU utilisation over 25% when playing a YouTube video.
There has to be something happening behind the scenes which is causing such a problem that doesn't appear on my computer - what version of Flash do you have installed?