True HD is 1.2 - 1.5Gbps (yes, that's a G), so for any Internet TV service to claim that they can deliver this (let alone for anyone to be able to view it) is misleading. But, of course, not even broadcasters can manage this data rate, so the content gets compressed for transmission on cable, satellite and digital terrestrial.
The reality is that it's the resolution of HD that is being used, with slightly higher data rates (and costs) than normal Internet TV.
Indeed, HD is in the eye of the beholder. I've seen some dreadful HD pictures on my home service - especially for sports events, and some great pictures for drama content.
So, HD is something to be aware of , both for its cost (a 2Mbps feed obviously costs four times what a 500Kbps costs) and for what it offers.
I'd argue that there's a call for a HQ definition for Internet TV based on subjective measurements - fundamentally the quality of the video, not the technical specifications.