Amit Klein posted the following to the web security mailing list yesterday.

"User tracking across domains, processes (in some cases) and windows/tabs is
demonstrated by exploiting several vulnerabilities in major browsers (Microsoft Internet Explorer, Mozilla Firefox, Apple Safari, and to a limited extent Google Chrome). Additionally, new cross-domain information leakage, and cross domain attacks are described, which provide a foundation for attacks such as “in session phishing”.

According to Opera’s security team, Opera is vulnerable as well, but it was not researched by the author.

Apple released Safari 4.0 today, which contains a fix for this problem." - Amit

For those of who you aren't familiar with Amit he always puts out top notch
stuff and is someone to watch within the web security world.

Download:
http://www.trusteer.com/temporary-user-tracking-in-major-browsers