Apple has replaced 3 iPhone 4's for me all due to the slide to unlock not working. This last iPhone 4 I received at the genius bar came with 4.2.1. For 3 months the phone performed flawlessly, but I secumbed to 4.3.3 when it was released and imediately after the install the slide to unlock was NOT responsive.
Not sure what this says other than it seems to be either a 4.3.3 issue OR something is corrupting the software updates in iTunes.