I mentioned this in a response to a different thread, but yeah, exactly the same issue.
My home button used to be bang on but since installing iOS 5 it's sometimes unresponsive leading to the issue you mention above or sometimes it just doesn't close the app. I click the button but nothing happens...
It's infrequent at present but it is there and I've got a feeling it's gonna get more regular
One of the other members suggested it could actually be a hardware fault. My button wearing out etc, and that apple would potentially do something at the genius bar
Sent from my iPad using Tapatalk