Right - I will!
Tonight I picked up my "warranty" replacement ATV2 from the local Apple store, which had 4.4.4 installed by default. I am happy to report it works perfectly again. My problem was exactly the same as the original poster - "stuttering". So I think infinityplusone can fix his/her problem by the same means - the timing was just too coincidental, that two people suddenly complain about a 3008WFP not working days after iOS 5.0 came out.
The ATV2 with 5.0 DID work fine with a different HDMI TV. So clearly it was the OS. I suspect Apple is too big a company to care very much - most people will be using it with TVs.
The very first thing I did was download seas0npass and tinyumbrella to jailbreak and back up the SHSH in case it "accidentally" updates itself sometime. Isn't it stupid that I had to do a "warranty" replacement to fix this?! Apple tries to stop you downgrading - they are not the Apple of yore. I was even thinking of getting a U3011 (successor to the 3008WFP) in order to get 2 HDMI ports to connect my PVR/Blu-Ray player and Apple TV to separate ports, and maybe getting an ATV3, but now it should be sufficient to get an HDMI and audio switch.
I also have to say I can't see any advantage of 5.0 over 4.4.4 - I'm from Australia and don't care about MLB / WSJ / NHL at all. One thing 5.0 did was just make those even more prominent. I suspect many Americans are the same. Now I can install XBMC and watch al-Jazeera as I wanted to - don't need Raspberry Pi just yet.