In reality, iCloud backups (and for that matter, iTunes backups) are not like the "average" backups we're used to in the PC/Mac world.
They don't contain the OS. Just settings, media, app data and that's it.
So for instance you can start your new iPad as "New Device", jailbreak, then restore from an iCloud backup and you'd still be jailbroken.
Only restoring firmware (and I'm not sure ir erasing all content) will get rid of it.