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.