I don't know about your question, but I was choosing between the Dell 24" and the ACD 23" - and went with the ACD.
Why?
1) Not needing analogue components, I didn't want the heat and unreliability they imply (all parts have a chance of failure; the more systems, the most chances you have to lose)
2) the ACD is sealed thanks to its lower heat output (and thus lack of a need for active cooling). This means no dust etc
3) Unity of Warranty - Apple services both computer and display, one less entity to deal with
4) Dell service - I had a dell laptop 2001-2004, and I kept having to send it back to them to fix LCD-related problems. I realise that there is a low chance that the Dell 24" would have the same problems, but I have a firm belief that if it did, Dell would give me the same runaround, and would not be able to solve the problem on the first try (or the second, or the third...)
5) local service options - There are 3 (soon to be 5) Apple stores within 15 miles of me. I can take the display there to be serviced rather than needing to ship it to some flyover state, with no assurance of a warranty response.
All in all, I'm well satisfied with the $350 margin between the two for the benefits I get out of it. (ACD 1299 (edu) vs Dell 950)
-RS