Those tricks are old, and are indeed in use for access at porn sites, as described. Captchas won't keep them all out (as you notice around here), but they are not worthless, either. After all, it DOES create a minimal barrier that might stop half the spamscripts. Without it, the flood might be a lot bigger.
I kinda like the ones where they ask you a question like 'how much is 1+3?', or even the one that asks you to enter 2 random big numbers and uses it to calculate pi (poke around on <!-- m --><a class="postlink" href="http://www.ridiculousfish.com/blog/">http://www.ridiculousfish.com/blog/</a><!-- m -->.