Well that's clear as mud, isn't it?
Apple points to a knowledgebase article here, indicating how much instability you might expect after jailbreaking your iPhone. Of course, now that jailbreaking is legal, Apple can't really go after anyone doing it, but that doesn't mean they have to support your functionality habit, either. Because, as Apple has vaguely worded, it might void your warranty. Not that it will -- because if you don't mess with baseband and restore your phone, no one would be the wiser. A simple complete restore really does bring your iPhone, iPod touch, or iPad back to factory conditions.
I'm not sure what harm jailbreaking an iPod touch is going to do to AT&T's precious network, frankly (considering there's not even a cell radio in the darn thing). But hey, it's Apple's product and they can, or cannot, void your warranty if they want to. We just don't know if they will. We'll have to wait for the Imperial Veil of Secrecy to lift yet again.
Keep in mind you may jailbreak your iDevices without unlocking them. An unlock allows you to use different carriers, whereas a jailbreak will allow you to run software Apple hasn't approved. I see the logic in their statements about jailbreaking and stability. No manufacturer is going to claim someone else's work is better than their own. Certainly not Apple, not in this universe.
What I don't get is why Apple fails to deliver clear answers on important questions for consumers. Perhaps someday they'll release the iTapDance shoes they've been wearing all these years.