Yeah. If you want a screen thats exactly the same as the 27" imac, the only screen you can get is the U2711. I made the plunge and I'm relativity happy. They pretty nice together but it would be nice if Apple came out with a 27" cinema display.
The dell uses exactly the same UPS Panel as the iMac so colour is almost exactly the same once calibrated (I had to use the imac calibration to colour looking the same). The only thing is the U2711 doesn't use LED backlighting like the iMac, so its not as bright.
But in terms of not wanting to use a second screen thats not the same same size/resolution, I totally understand and I haven't regretted getting my Dell.
The other option to the dell is you could get a second 27" imac (lowest spec of course) and use it as a second screen. Other than being EXACTLY the same, it means you've got a second computer as a redundancy.