When I tap on the bottom of my iPhone's screen it actually acts as if I tapped in another section higher up. It may be a calibration problem. How can I recalibrate the touchscreen?
The iPhone's capacitive touch screen doesn't have the concept of calibration.
To fix the problem you're experiencing, I suggest trying each of the following options:
Hard reset of the phone by holding the Home and Sleep buttons for 10 seconds.
"Reset All Settings" from Settings → General → Reset.
"Erase All Content and Settings" from Settings → General → Reset.
If your phone is still under warranty or covered by AppleCare, bring it to Apple and they will repair or replace it for you.
I just recently had this problem. After some panic, and consideration of replacing my digitizer, I calmed down and looked at my phone carefully.
Keep in mind that an iphone works on capacitance, with the glass plate being an insulating layer just on top of the sensing array.
On inspecting my phone, I discovered that the bottom right corner of the glass face(to the right of the home button holding the phone in portrait orientation)had become flush with the top of the chrome strip which runs so elegantly around my 3g. It's not supposed to be flush. The glass top on a 3g/gs should be slightly higher.
I powered down and removed my sim card, removed the screws at the bottom and re positioned the face properly.
Good news? I returned to proper working order. Bad news? it doesn't last. Most likely I will have to replace the digitizer.