Asa Raskin's developed a fairly ingenious, CSS-based means of obfuscating text; briefly, he inserts random characters in the text and applies a "do not render" CSS style to them. The words render as normal on your screen, but when copied to the clipboard, the junk text is also picked up.
It's been years since I did much with CSS, but I have a feeling that you could create a bookmarklet that calculates what the text should look like as rendered and discards the hidden characters, but I fully expect the DRM snake-oil peddlers to leap on this as a way to make "uncopyable web-pages."
Besides spammers using this trick to get around your Bayesian spam filters, there are other bad things for which this can be used. This first is for that misguided holy-grail of publishers: copy-protection for their words. A publisher could generate, on the server side, a new random mess of HTML and CSS that would render their text uncopyable. This also has the side-effect of making your pages impossible for search engines to index sensibly; it's an easy way to keep your information human-readable but cloaked from Google's all-seeing Sauronic eye.
How To Phish, Protect Your Email, and Defeat Copy-And-Paste with CSS
(Thanks, Alan!)
