Any idea if the next Cinema Display update will include a 21" or 23" version?
(PS - do you mean 'Cinema Display' (Displayport) or 'Thunderbolt Display'? I don't imagine the Cinema Display will ever be updated).
Anybody's guess - however, thinking about it, the 27" displays have always been promoted primarily as a MacBook Pro accessory (hence the built-in magsafe). With the MBP line likely to shift entirely to retina (maybe not this spring, but in the not-too-distant future) maybe Apple will have a rethink.
If Apple
were to go for an external 'Retina' display, they might drop to 21" rather than trying to find a 27" panel with enough pixels to justify the 'retina' tag.
They could also do with a smaller/cheaper TB display for Air users.
(All speculation, of course)
Also, when in the world are they gonna put a HDMI/DVI input?
Next time the Winter Olympics are hosted in Hades, I guess. Even if the proverbial Apple TV set appears, I wouldn't bet on it having HDMI.
Meanwhile there are nice, relatively cheap, 24" 1920x1200 displays from third parties with DVI and/or HDMI. Just do a quick check if you're going to rely on using the Displayport input on a third-party display as some have problems with Thunderbolt Macs - although you can always fall back to a Mac-to-DVI or HDMI converter. I have a Dell U2412M (Displayport + DVI) that works fine over Displayport or DVI.
Personally, for anything smaller than 27" 1440p I prefer a 16:10 screen to a 16:9 ratio (i.e. 24" rather than 23") - space for 2 A4 pages @100% plus a top toolbar. Black bands when watching 16:9 video never killed anybody.