When it was subtly footnoted that iPhone 3G and iPod touch G2 wouldn't be getting iOS 4-style wallpaper, our only guess as to why was CoreAnimation performance -- Apple couldn't do their transitions over the textured background at levels that met their auteur standards.
Erica emailed Steve Jobs to find out:
Hey Steve! I just upgraded my iPhone 3G to iOS 4 and was really looking forward to setting a background on my home screen. Guess that's not happening, but I'd like to know why.
And Steve Jobs replied:
The icon animation with backgrounds didn't perform well enough.
Jailbreak users have been theming since before there was an iPhone 3G, of course, just like they were recording video and multitasking, but Apple won't put their brand on anything they consider sub-par in the performance area.
So there you have it.