In the case of the 30", the prices aren't THAT much different, though; the 3007WFP runs $2200 from Dell, and the 30" ACD is $2500 from Apple. They appear to use the exact same panel, so it really comes down to style and features. The Dell has 4 USB2 ports instead of 2, and it has a flash media reader, but it doesn't have a built-in FW400 hub (I've found that handy for connecting a DV camera).
Styling is totally a matter of personal preference (I really love the thin, brick-on-floor ACD design, but others don't), but the ACD has no lights on the front and a grey bezel instead of black--one or the other might be preferred depending on the environment. The single cable from the ACD is sure nice, though.
Worth noting that I've read about people having problems getting their Mac to sleep with a Dell monitor plugged into it because of the integrated media reader--it apparently keeps waking the Mac up via the USB connection. Disconnecting that of course makes it ok, but it's a possible annoyance.
The Dell comes with a 3 year warranty, which you're going to have to pay $99 for from Apple unless you buy it along with a Mac that you're getting AppleCare for. For some extra cash, you can get a Dell warranty that covers up to 5 years, and/or even covers accidental damage.
Last question (and it really is just a question): The "Customize" page for the Dell lists a couple of $100 Belkin DVI cables--I assume it already includes a cable, but I can't find where it says that.
[Edit: Last note: I just noticed you can get a refurb 30" ACD for $2099, so even with the extra $99 for 3 years of AppleCare, it comes out to the same price as the Dell that way. If I had $2000 laying around to spend on a monitor, I'd be seriously considering that option, just because I'm so fond of the ACD styling.]
