I've been playing with several apps, so I'm not that perturbed -- yet.
I'm now going to try rebooting it each time I put it in the night-time charger:
Apple - Support - iPhone - Phone TroubleshootingShould clear out some gremlins. It's an old Windows trick.
To reset iPhone, press and hold the Sleep/Wake button and the Home button at the same time for at least 10 seconds. During this time, a red 'power off' slider may appear and the screen may go black, but do not release Sleep/Wake and Home buttons until the Apple logo appears.
Incidentally, here's how to kill an app (If I have to do this, I'll reset at the next convenient time):
Try pressing and holding the Home button for at least 6 seconds to close a frozen application. Then restart (power cycle) by press and holding the sleep/wake button until the red slider appears ...