This morning, Laini called me over that there's something wrong with her iPad. Every time she launched the iTunes app (she has a $10/month allowance and today it refilled) it opened the App Store app instead. I had her bring it over to me, and sure enough, click "iTunes" and it flashes for a second and switches over to App Store, using the same animated motion as I see on my iOS 4 iPhone when you use fast app switching.
You have launch iTunes and quickly press and hold the music button before it switches. If you catch it in time, iTunes will open normally from then on. No idea why it happens but that is what I did and been fine since.