Again, to notice the difference between TN and IPS you need to have two displays set up side by side. It's not something of any sort of magnitude.
Not necessarily the case with everyone. Most people can see the difference by looking at them. The TN panel will be noticeably darker due to the higher contrast, and depending on the resolution, may be a bit softer.
As far as my knowledge goes, the ACD and Dell UltraSharp 2005FPW, use the exact same panel inside: an LG Phillips LM201W01. They use different backlights, and the Dell has four USB ports, but essentially they are the same LCD.
Unless you're going for the all-Apple look and willing to pay the premium, get a much cheaper Dell (or any other brand for that matter) screen with the same/similar panel inside.
This is true. The only thing separating the Dell and the Apple is that the ACD is SWOP certified, again, needed for accurate color and professional (non-gaming TV viewing) tasks, and the backlighting is different.
The 23" and 30" ACD are also SWOP certified while the Dells aren't, and the Dell 24" 27" 30" all use S-PVA or TN panels, while the Apple's are still the top of the line S-IPS. Both are made by LG. The big problem with an update of the displays are the panels, since LG nor Samsung make any newer professional panels, there is nothing to upgrade to.
The only thing Apple could do is release a cheaper TN panel with a plastic bezel, or change the bezel on the current models or lower their price for a fourth time.
Again, in the professional world, there isn't anything that really competes with the ACDs other than the NECs and LaCies that come with color calibrators and slightly better casing. They are cheaper than the ACDs, and still give the superior panels.