Cannot "slide to unlock" when iPhone is in dock. Any idea why?
Strange issue...

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.