Unlocking voids the apple warranty unless you manage to restore it before you send it in for service, and even then there are changes made to certain files that are irreversible and should they decide to look, they will see that the phone has been unlocked at one point. I've seen situations in forums where people send in unlocked phones for hardware problems and Apple decided to see if the phone has been unlocked, and considered the warranty void, period.
The AppleCare for iPhone plan says this about situations not covered:
"(ii) Your use of or modification to the Covered Equipment, the iPhone OS or iPhone Software in a manner for which the Covered Equipment or software is not intended to be used or modified."
Later on this is addressed again:
b. Limitations The Plan does not cover:
(ii) Damage to the Covered Equipment caused by accident, abuse, neglect, misuse (including faulty installation, repair or maintenance by anyone other than Apple or an Apple Authorized Service Provider), unauthorized modification, <<<<<< unlocking is considered unauthorized modification.
So should you unlock and one day your phone is completely bricked, you're out of luck. You get to buy a brand new phone.
If you have a hardware problem-- volume buttons get stuck or something-- and can restore the phone to factory defaults (if an iPhone 4 unlock isn't permanent) then you might still get warranty service if you get the tech who's going off shift in an hour and just wants to go home.
So it's a gamble. You might get lucky, or you might end up having to go to a third-party "repair service," or even buy a new phone outright. I say if you're going to unlock, don't even bother with the APP, the iphone comes with a one year warranty anyway.
