Bill Zeller and Ed Felten have an interesting paper, "Cross-Site Request Forgeries: Exploitation and Prevention" (PDF), that looks at exploiting the implicit authentication in browsers to take actions on the user's behalf using img tags or Javascript.

The most dramatic of the attacks allowed the attacker to take all the money from someone's ING Direct account just by visiting a web page. The attack sent POST requests off to ING Direct using Javascript, so they appear to come from the victim's browser. The POST requests quickly and quietly cause the victim's browser to create a new account by transferring money from their existing account, add the attacker as a valid payee on the new account, then transfer the funds to the attacker's account. Danger, Will Robinson.

Please see also Bill Zeller's blog post describing the attack and the Wikipedia page for cross-site request forgery.

[Paper found via Bruce Schneier]