It'd simply be the digital equivalent of the stickers that indicate if a unit has been opened or exposed to moisture.
Apple's "it's a crime" talk is to scare casual users away from jailbreaking to get pirated software. But they know that experienced users, the ones that write the apps that draw the inexperienced users to the platform, want to jailbreak. And, those users want a warranty. And, they don't want to piss off those users, many of whom have hardware-review blogs.
So my guess is that Apple is not going to void warranties for jailbreaking. If they start doing that, it's going to be really good for Android and really bad for Apple.