Borrow someone's iPod or iPhone, with the Home screen showing do a screenshot, jump to Photos and show this new 'photo', then hand it back to the owner telling them you are having problems choosing an app.

UPDATE: Take a screen shot of ALL of the Home / button 'screens', left to right in that order so they are in that same order in Photos when you finger-swipe them (it'll behave just like the regular Home/menu buttons do!), then start on the Home screen in the Photos, handing back the iPhone/Touch to the owner. This will add much more (initial) confusion to the owner!

Now, you can do this in opposite as the owner, too! Take the screen shots and have them ready and in the waiting.
Hand the iPhone/iPod Touch with the actual Home buttons in view to someone to let them try something out (brag about a feature and/or app that they 'just hafta try out'). After a few seconds ask for it back "to do something you forgot" ("Oh, I forgot to send an important email, lemme borrow it for a moment.")
Jump to the Photos and show the screenshot Home screen. Now, hold on to the iPod/iPhone for a bit longer, as this 'extra' time will 'cover your tracks' in that you are actually doing that 'something'.
Then, hand back the iPod/iPhone back, say "Sorry 'bout that. Go ahead" and observe as they then try to relaunch the app they last chose.
Of course, then begin questioning what they 'did' to 'freeze' your iPod/iPhone.

Also, you do this to a Genius at the Bar in an Apple Store. Start by showing that you have a problem (that apps won't launch), yet on the actual home screen they do. Ask for the iPod/iPhone back to 'try again', but this time switching to the screenshots, then say "See, it's doing it again" handing the iPhone/iPod back to the Genius and see how quickly their 'genius' takes to figure things out...

I found a screen shot from Tap Tap revenge... I was wondering how I did it and now I know!!!!