HD means the resolution is equal to or greater than 1080 lines.
The 30" display has always been higher than this (1600 lines) and as such can be considered HD.
You cannot plug an Apple Cinema Display into a Blu-Ray player because the DVI connection on the screen does not support the HDCP copy protection protocol. High Def videos from iTunes (trailers, clips etc) will play fine so long as your machine is fast enough to play it but HD videos from an optical disc won't play back to their full resolution.