According to the manufacturer, maybe. Intrepid Apple blognaut Leander Kahney rang up Apple, asked the question above, and got a response. This was more than we got when several of us asked Apple multiple variations of the same question. Apple's statement, as seen on Cult of Mac:
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.