http://userscripts.org/scripts/show/25846 does this for any site you add @include rules for, and also line breaks it nicely to fit the layout in as few lines as it can.

This script removes the pop-up (tooltip, title="") from XKCD's comic image and shoves that text below the comic.

While the initial purpose of this script was a workaround for Mozilla's bug 45375, that bug was fixed in Firefox 3.

Even after the release of Firefox 3, I find this script's solution preferable to Randall's tooltip quips.

Some people think it's too easy to accidentally read the text before finishing the comic, and there are similar scripts that make the text white-on-white (requiring you to select it) or otherwise hiding it, but this is the simpler solution, and I'm sold.

Update 2010-01-21: Due to the fact that this will draw the title before the image if your internet connection is a bit slowed down at that time, I've rewritten this script to ensure it waits for the image to load so you don't have to risk the spoiler.