I tend to prefer the aluminum frame color used on the Cinema displays and MBPs, simply because the color is a pretty neutral match to the silver/gray used in Apple's pro applications, like Final Cut and Aperture. To me it feels less intrusive and brighter.
However, from a media standpoint, like for watching movies, I prefer black. I like watching movies on my black-bordered Dell monitor, because it just sort of disappears, especially with the lights dimmed. It's also the reason I bought a black iPod video instead of a white one.
The iMac is an interesting mix of these two options. I think Apple made a good choice with the seamless black border covered in glass on the new iMacs, since it's primarily a media/iLife machine. iLife '08 seems to use the same color scheme, and brings more black into the apps.