To understand the gist of this joke you'll have to first watch Apple's new "there's an app for that" commercial at You Tube or from the official Apple website.
After watching at least a couple of those commercial, head on to the App For That website that gives you a series of user submitted “App For That” jokes, which you can vote on, tweet out, email, etc. The service looks like the iPhone’s text app, with submissions written in chat bubbles. To refresh the page with the latest joke, just hit the refresh button in the middle. And if you'd like to submit your own jokes, there's a form for that at the website.
Too bad the comic value of this joke will be gone after the first half an hour or so.