Just thinking this morning, after reading most of what was said over the last two days regarding the desktop Macs. The articles and Apple have clarified that the Mac Pro is being redesigned in 2013, not the iMac. This helps us out to speculate with some reasoning.
- The portable line was fully refreshed this week. All in sync with similar specs/options [Ivy Bridge, USB 3, TB] iMac and Mac Mini - the last 2-4 updates for both have been either at the same time or within a month or so, so they are basically in sync, sort of. Mac Pro is on its own really, and the audience is a bit different anyway.
- I see the iMac and Mac Mini updating in one month [July 2012], shipping with Mountain Lion [no technical reason though]. This delay is to simply not take away from the chatter and reviews of the new MacBook Pro Retina. Simple as that. It is about maximizing the press on the portable line, then press releasing the iMac and Mini in July to coincide with ML. [with Ivy Bridge, USB 3, Intel 4000 graphics and possibly updated discrete GPU in appropriate models. Maaaaayybe new SSD/HDD options] No Retina, nothing else really. Parity with the portable lineup.
- Retina displays will debut across the portable line over the next 1-2 years, then make their way into iMac and TB Display in 2013-14.
This is just a hunch, but I feel that it is this simple. No malice or spitefulness on Apple's part, just maximizing the press exposure for the latest newness. Mac Pro, well, maybe a quiet option upgrade before the redesign, like an updated GPU or if they figure out TB on PCIe card.
Again, just a hunch, but I feel pretty good saying it.