You can read more about it and download the code here: http://lyncd.com/2008/11/minify-for-wordpress/. (And view source to see what it does!)
I couldn't determine a way to get it to work with Donncha's plugin hooks, so right now it's just a separate class and a patch to 2 of the WP Super Cache files. (And I just put everything in one tarball, if that's what you're looking for.)
Hopefully you'll find this useful (or at least interesting). Feel free to post comments/questions here, but I'll get to them sooner if they're posted here, since the blog pings me.
Note: It does require PHP5, since Minify is written in PHP5, and PHP4 is teh suck.