They dont' fix it. The "Genius" will check the phone to confirm that it is a warranty issue and if it a covered problem they will just hand you an Apple refurbished iPhone 3G.And they either fix the phone or replace it on the spot samy day ?
Do you have a specific reason to take one in for warranty exchange? Please remember that they don't cover water damage (liquid) or if you drop it and break the glass or the LCD.
Dave