*"I believe this isn't always the case. If you're using multiple browsers in OS ...

"I believe this isn't always the case. If you're using multiple browsers in OS X all built on Webkit for instance, they share the same cookie store."

If that's true then it's utterly insane. This StackOverflow question from 2008 seems to confirm what you're saying, though:

http://stackoverflow.com/questions/364219/how-can-i-have-mul...

I really hope it isn't still the case...