I have a third-party iPhone dock connected directly to the mains. I found that I cannot unlock my iPhone 3GS when it is in the dock. The slider just slides back and doesn't follow the finger's movement completely.
Any idea why?
I would guess that the dock is somehow interfering with the capacitive 'field' of the screen. You could confirm this by unlocking the device before connecting it to the dock and see if the rest of the interface is affected in the same way.
Though I guess the only way to proceed would be to get the dock replaced (if it's under warranty) and see if that fixes the problem or buy a different one.