Safari 4 also fixes an interesting cross-domain XML theft. Full technical details live here:
XML theft can include highly sensitive data thanks to things like XHTML, AJAX-y RPCs using XML and authenticated RSS feeds. The example I have steals XML representing a logged-in Gmail user's inbox:
Safari 3 demo for users logged in to Gmail
I think there's a lot more room for browser-based cross-domain leaks (sometimes called UXSS or universal XSS). This is because the pace of new browser features is very high, and lots more functionality is being added that involves reference by URI. Every such addition is a possible vector for a missing or incorrect (e.g. 302 redirect tricks) cross-domain check; or even an ill-advised specification-based cross-domain leak.
This is one of the serious Safari bugs demoed but not disclosed at my PacSec and HiTB Dubai presentations. I forgot to note that my previous post on file theft was another.