Well, now that H.264 is out and about, you could use a 23 to watch 1080 video (except the 12 doesn't play it back very well...see my post).
And given that more video will be made available in the near future in HD, you could use your display to watch it. Of course, realistically, widespread 1080 is likely a year or two off, so you might hold off on a 23 until you would really want it for HD because they'll be a lot cheaper then.
I have a 1.33 12" and a 20 cinema at home. It runs it just fine, I don't notice any sluggishness from the monitor, and its plent of real estate for me (I generally use VectorWorks, Matlab and occasionally Photoshop and such).
I also have a Dell 2005FPW that I have at work and actually I like it better than the Cinema. It uses the same panel, so the picture looks identical, it can tilt, rotate and lift, it has picture in picture and a whole host of inputs. And, with randomly available coupons you can get it for $350.
Unless you are deadset on the matching case, I'd go with the Dell. It is cheap as all get out and you can always use it for a year or two, and then if you find you are wanting for HD you could spring for a 23 Cinema, which would be much cheaper by then and maybe even Apple will add an input so it could be used as an HDTV.
There is a nice comparison between the Dell and the Apple
here.
Here is a forum where they keep track of online deals and the 2005FPW is one of the
hot items