Safari 4 runs plug-ins in separate processes. That didn't seem like a compelling feature ... until I discovered
BashFlash, a tiny utility that monitors Flash usage and gives you the ability to kill a run-away Flash plug-in.
Running Flash in a separate process would also avoid Flash crashing all of OmniWeb. I can't count the number of CrashCatcher reports I've submitted where I've had to disclose my YouTube weakness.
-- Ward