I just came home a little while ago from the Apple Store with a new 24" LED Cinema Display. I'm going to be using it with an iMac until I get a MacBook Pro. The problem is I cannot plug it into the mini display port on the back of my iMac, it doesn't seem to be matching up for some reason. I know I'm using the right adapter because the mini display port cable on the display, and the port on the back of the iMac have the same icon. Doesn't seem to make any sense.
Your iMac doesn't have a mini displayport, it has a mini-dvi port. There's no adapter for mini-dvi -> mini-displayport as far as I know. There's a company working on one but I don't think it's out yet.
Icon matching does not matter. The icon you see is the generic Apple icon for video (vertical bar, square, vertical bar). It does not indicate anything about what type of video port it is. As the previous poster mentioned, unless your iMac is the absolute latest revision, it has a mini DVI port. The ACD need a mini DisplayPort connection. Time to return the ACD ... and I can buy it as a refurb