Update: Mozilla have now decided to disable IDN support by default in their browsers.
More info here:
http://slashdot.org/article.pl?sid=0...&tid=154&tid=1
EDIT: I've never seen one of these URLs before, but in case you're wondering what it looks like...
http://vÃÃâ€*’Ãâ...€šÃ‚¤vtak.se/ (doesn't work in IE)