Creating a post in Discussion only requires “a few” points of karma; creating one in Main requires 20. I believe 20 is also required for creating a Meetup post; in most ways those appear to be treated as posts to Main (for example, up- and down-votes on them count for 10x the usual amount of poster karma).
Source: The LW FAQ, specifically http://wiki.lesswrong.com/wiki/FAQ#Why_do_I_want_high_karma.3F
That’s worth checking (both in terms of what Apple claims, and in terms of what any relevant legal precedents claim; a hardware warranty certainly shouldn’t be at risk from a software modification). On the other hand, it should be easy to “un-jailbreak” a device; just restore an un-jailbroken image onto it (for example, from a backup made before jailbreaking), and you can do so before sending the device in for warranty service. If the device is “bricked” to the point that you can’t restore it, then Apple probably can’t tell that it was jailbroken, either.