Sometimes a app just hangs and doesn't respond anymore. But a full reboot of the device takes some time and is maybe more than would be needed. When something like that happens I try a reset first. Thats the equivalent of killing all tasks in a taskmanager. It's super fast and solved my problems in 9 out of 10 cases.
On the old 2G and 3G this worked by simply holding down the Home button for about 5 seconds. The device jumps back to the Springboard and everyone is happy.
But when I realized that this didn't work on the 3GS anymore (because holding down the home button opens the voice rec software) I was pretty annoyed at fist. Until I found out that Apple changed this in the 3GS. On the 3GS you have to hold down that Wake/Sleep button until the "Power off" slider appears and than hold down the Home button (let go of Wake/Sleep) until you're back on the Springboard.
Apple really has a habit of keeping those little tricks a secret. I would love to see a small handbook or introductory app coming with every iDevice where you can learn all those little gestures and routines.