From the screenshots I've seen of FibreOp TV, it's not Mediaroom. MTS, Telus, Bell and Sasktel have Mediaroom, so Aliant is the only major Canadian telco not using it - yet.
I had it for a couple of days when it first came out and found the pixelization a lot worse then Rogers.Fibreop runs on MPEG-4 and I see no issues with HD quality, i find it better than rogers which was highly compressed and often has pixilization