I think its a good rule. Developers should take pride in their work and make their app reflect the quality of their product. Its sad that Apple has to enforce this (which should say something about the Dev's who have it done it's products.)
I can understand that but what about the dev who say made the app a long time ago before the 4 came out or shortly after the 4 came out. All he had was say either a 3GS or iPod for testing and taking easy screen shots and much better screen shots than could be grab off the SDK.
All said dev wants to do is update some bugs in the app but lacks the tools to really do a full retake of the screen shots.
I think the rule is good for future apps but making it Apply to updates I think can be an issue as it can increase the work load on older apps just doing a basic patch.