I got this once too, but then tried again in a few minutes and everything was ok. Not sure if it's a bug, too many people logging in at once, or what...
My solution:
1) turn off the device
2) go to itunes, in the menu to store -> authorize computer
3) enter your apple ID to authorize the machine again
4) turn on the device
5) sync your apps
Hope it works. It worked for me. 🙂
it has happened to me before on two different applications. It was on aim and poker. I tried the home button for 10 seconds and it did not work. Tried waiting it out and it did not work. I had to shut down the phone holding the power button for 10 seconds and boot it back up and the application worked.