Stuck in a calendar on iPhone
Double tap the start button, and it will bring up a list of running programs.  
Press and hold one of them, and they will start to jiggle, and show an - icon in the upper left corner.
Find the calendar program, and tap the -.
Rinse and repeat for any other apps you're having trouble with.  I like to periodically go through the list and close down apps I'm not using/won't use for a while


NOTE: Do this from the home screen. If you are in the app it won't appear in the queue of running apps.