In Snow Leopard, Safari 4 will run Flash as an separate process, so that if/when Flash crashes, the rest of Safari won't be affected. You need to be running Snow Leopard, though; Safari 4 in OS 10.5 Leopard still runs Flash as a plugin, and Flash crashes will still crash the entire browser.
Here's a screenshot from the page "Apple - Mac OS X Snow Leopard - Refining the user experience":
Moreover, here's a quote from this press release from Apple:
In Mac OS® X Snow Leopard™, available
later this year, Safari runs as a
64-bit application, boosting the
engine by up to 50 percent.** Snow
Leopard makes Safari more resistant to
crashes by running plug-ins in a
separate process, so even if a plug-in
crashes, Safari continues to run and
the user simply has to reload the
I can't speak to why this new feature is 10.6-only, but it seems pretty clear that it is.