Via a bug in Firefox's Error object: http://33bits.org/2010/06/01/yet-another-identity-stealing-b...
Via a bug in Google spreadsheets: http://33bits.org/2010/02/22/google-docs-leaks-identity/ (I found this one :-)
Via history stealing: http://33bits.org/2010/02/18/cookies-supercookies-and-uberco...
More sophisticated, but hypothetical version of previous: http://33bits.org/2010/02/19/ubercookies-history-stealing-so...
XSS bugs and other problems with Instant personalization partner sites: http://33bits.org/2010/09/28/instant-personalization-privacy...
I've also been predicting that this will eventually become the new normal -- both because the bugs are coming too fast to fix (and exploits in the wild will become more common) and because Facebook is pushing to change people's expectations with Instant Personalization.
The other day I attended a talk about one-click frauds. I realized that that's the perfect black-hat use-case for this class of attacks (although current 1-click fraudsters are apparently rather low tech). Stay tuned.