This is a panel from a webcomic I do. If you like this, go check it out: I Was Kidnapped By Lesbian Pirates From Outer Space!!!
I read a similar tutorial a few months ago, but when I tried it, it didn't work. So I messed around until I found something that worked for me.
If you want to follow along, I put up the elements for the tutorial here
So, this tutorial requires a bit of pre-existing Photoshop knowledge. The alpha channels are usually the biggest hurdle. But once you figure them out, you'll be using them for everything.
EDIT [February 16th, 2010]: I've been learning a lot more about CMYK as I've been using this technique more and more myself. The red type contains my tips for making this EVEN BETTER!
And because so many people have asked, Photoshop is the only thing I know of that can do this. And it works best the larger the image is. I work at 450dpi, 7" x 10".
EDIT [February 21, 2010]: I recorded a video of myself using this technique! [link]