Android phones are delivered with proprietary apps that greatly improve the user’s experience. Some might even say an Android device without Google apps is only half the fun.
For being accepted as valid packages, apps are signed. Moreover if an app wants special-1337-system privileges, it has to be signed with the platform key. This is to make sure apps behave correctly.
So if you come across a message that tells you the system just ignored a package (“Package xyz has no signatures that match those in shared user android.uid.system; ignoring!”), just resign it with your platform key. If you don’t have the platform key: Bad luck.
This is rather a note to myself (the sky just fell on my head):
Google apps that always need resigning are GoogleCheckin, GoogleSubscribedFeedsProvider and NetworkLocation.
Don’t ever dare to sign any other proprietary app, Google doesn’t like that and your Android system won’t allow you to use it.