I've had this happen a couple of times now and the first time was a little worrying that I'd bricked my iPhone. Basically I was running an application on my phone via XCode and when rebuilding an updated version it failed with a "busy" error message. Stopping XCode and unconnecting my phone had no effect, the phone was stuck displaying the loading screen of the application and wouldn't respond to any key commands. To fix you have to hard reboot, holding the power and home button until the phone reboots - doesn't lose any of the data you have on your phone (a concern the first time I did it).
2 weeks ago