You can have it chop off the end of the URL with "overflow-x: hidden" and an explicit width, but the only way to have it insert a linebreak is to count characters on the server and insert a break manually.

You can use a 'zero width space' (​)in some browsers to hint a breakable point, but it might affect C&P-ability.

Some other discussion of options here:

http://gojomo.blogspot.com/2005/03/cross-browser-invisible-word-break-in.html

Thanks, 200b is was I found too after reading the CSS 2 spec.