Mail wraps URL's longer than 80 characters, so that they no longer work correctly when a recipient clicks on the link. I know that you can use TinyURL to mail shorter URLs (see this previous hint for automated ways of doing this), but there is another way to include long URL's which is easier, especially if you include several links in one email.
Just put the URL between angle brackets, i.e., like this:
<http://www.somesite.com/a/really/long/url>
When formed in this way, the link remains clickable for the recipient of the Mail message.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070112124954101