There are many ways to interpret your request for "is it HD".
If you mean "can it display 1920x1080 resolution content at full resolution?" then the answer is: The 24" iMac, 23" Cinema Display HD, and 30" Cinema Display HD can.
If you mean "can it display "HD" content at full frame rate?" then the answer is: Yes. All Intel iMacs are fast enough to display all formats of HD at full frame rate. For iMacs smaller than 24", they would scale down 1080 content, but it would play at full speed, and 1080 content still looks great scaled down to a 1440x900 screen. Not to mention that lots of broadcast "HD" is 1280x720, which can be displayed at full resolution on every display Apple currently sells (including the 1280x800 displays on the MacBook.)
Please note that 1280x720 still counts 100% as "HD". Most TVs sold today as "HD" are only capable of displaying 720p, and have to scale 1080 content down. And "capable of" to me implies "can it play" not "can it display at full resolution"...