Why invent an excuse? Liar
Only if it is really a defective one you deserve a good replacement.
These are the known defects:
-Plastic crack near the headphone jack
-sharp edge in the bottom part silver bezel not flushed with the plastic
-screen leaks, as a consequence it then will show dust grains inside your screen
-sharp edge/not flush SIM card holder with rest of the iPhone
This list is based on the 5 iPhone 3G that I have got replaced at the Genius Bar.
My current one has the plastic cracks around the headphone jack, but is barely noticeable only when it is exposed to good light conditions.
I have this problem and intermittent dead pixels (maybe not dead - perhaps stuck is a better description -but on occasions certain ones appear dead).
The bezel has bugged for a while but it never occurred to me that I could/should get it replaced.