This one took me awhile to discover, but you can show the Dock while in Lion's new full-screen mode:
Place your cursor to the side where you currently store your Dock (right, left, or bottom), so that it's sitting on the last pixel of that edge. Then lift your finger and push towards the edge again, holding it in the push. Voilà, the Dock appears.
Small thing, but I found this particular interface just brilliant. It prevents the Dock showing up accidentally, which could be a common problem in full-screen mode, given that, well, we're using the entire screen all the way up to that edge that normally triggers the Dock.