It turns out that as an April Fools stunt, Monday's strip serves up a different comic depending on your geographic location and/or the web browser and/or the device you are using to visit the site.
If I visit from work, or from home using Firefox, I get this:
If I visit from home using Chrome, I get this (click for full-size version):
If I tunnel my connection through a server in Seattle, I get this (click for full-size version):
The third one is peculiar in that it adds/removes entire panels of the strip depending on the width of the browser window; on the XKCD site you need to make your browser window at least ~1700 pixels wide to be able to see the entire thing, since the last couple of panels just disappear if the browser window is too narrow (no scrollbars). Also a bit creepy that there's a tunnel in the strip... did XKCD detect my use of a SSH/SOCKS tunnel somehow?
The mouseover text actually gives a hint that something fishy is going on:
XKCD #1037 wrote:Umwelt is the idea that because their senses pick up on different things, different animals in the same ecosystem actually live in very different worlds. Everything about you shapes the world you inhabit--from your ideology to your glasses prescription to your web browser.
If you visit from an AT&T mobile broadband connection, you supposedly get a page saying your bandwidth cap has been exceeded , and there are allegedly country-specific ones as well. Edit: It looks at referrer links as well; according to this article there's a different one for people who get there by clicking a link from reddit.
Post your version of XKCD #1037 (if different from the above)!