The iMessage/APNS initial registration protocol sends the hardware serial number of the device it runs on to Apple as part of getting its client certificate.

They're formulaic, so you can come up with valid ones that aren't your own, but good luck doing that in a way that scales well and doesn't run your app afoul of the DMCA.

I definitely did until I was sadly disappointed looking at the source.

hahahhahah I didn't think of that