wow...just keep calm down, it really so normal, nobody's iPhone and iPod Touch will show 100% battery unless plugging it into charge.
No that's incorrect, I can charge my iPhone 4 overnight and it will stay at 100% until I get to work about 2 hours after I unplug it most of the time.
I've never encountered this issue before, just wait and see if it happens again as if it happens again there may be a problem with the battery.
This has happened to me a couple of times after uplugging from a charge. Generally it went back up to 100% after a couple of minutes. I think it has to do with the charge controller registering a change in the current when the external power source is removed.
Bingo!!I think it also has to do with where in the charging cycle the phone is at the time you unplug it.