URL shorteners providing opportunity for spammers

I suppose equipping spam filters with crawlers that actually go out and classify the pages linked in an email, rather than just treating URLs (or parts of them) as regular words, would remedy this.

This has been proposed for a while (for example in http://www.paulgraham.com/spam.html). I'm actually surprised it isn't common yet, especially as more and more email tends to be hosted and processed by companies like Google.