I think you have your answer. In general, yes it will void your warranty. As far as iOS "phoning home" and tattling on you I don't think anyone is really sure about that. If they are doing that, I'm not sure how they are keeping that a secret.
Here is what Apple had to say about Jailbreaking (notice that it was updated last Sunday).http://support.apple.com/kb/HT3743
So in general, if Apple has solid evidence that the device has been tampered with they can deny warranty coverage.
Now, in practice, there really is not a whole lot of chit-chat out there calling Apple a bleaping-bleap-bleap for denying warranty coverage because a device had been jailbroken. Most people who jailbreak realize that taking a jailbroken device with a big fat Cydia icon on the home screen into an Apple Store to ask for help will probably be shown the door and I'm sure the incident will be noted (they have to explain everything they do with everybody in their customer ticket tracking software I am sure).
If you need to cover something under warranty, you simply restore your phone (this removes the jailbreak) and take it in.
That is exactly what I did in October 2011 when I took a previously JB iPad 2 into an Apple Store to check to see if they would warranty the device. They looked it over. They plugged it into something that looked like a diagnostic device and it ran some checks for something (?) for sometime (?). My complaint was about a nasty scratch that had developed on the screen that I was claiming was caused by the poor design of the smart cover. My unproven theory was that a small piece of dirt got between the smart cover and the iPad screen and festered. They looked the screen over with a pocket UV "black" light and then replaced my iPad 2 with a referb.
At some point, Apple may decide to implement a system that prevents iOS devices that have previously been jailbroken from receiving any warranty support or any Apple Store support period. It wouldn't surprise me. But consider this, the estimate for the number of people who downloaded and jailbroke their iOS 6 device was more than 1.7 million and that was just yesterday. That may only represent a little less than 1% of the total user base, but that is still a WHOLE lot of customers (most of which are repeat customers) that Apple probably does not want to anger.
I think that for the most part they will just continue the cat and mouse game.