It is well known that one can add a link to the reading list using the link's contextual menu. However, I have found another mouse shortcut for three-button mice. It works at least in Mac OS X 10.7.3 in Safari 5.1.2 and 5.1.3.
Click the link with the middle mouse button while holding the shift key and watch the link jump to your reading list.
[crarko adds: I don't have a three-button mouse handy to test this, but if you have a programmable mouse, you should be able to make an application-specific macro to do this too. If you try this out in 10.6, please post the results in the comments.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120219130531304