I doubt the upcoming iMac will be a "full re-design", but I could be wrong about that.
Nor will it be a simple "speed bump" (Ivy Bridge CPU) and addition of USB3. I'll -guess- that Apple is planning to introduce new display options as well. No retina display as of yet, but some kind of "matte" finish screen is likely.
I believe the main reasons for the "delay" are design/manufacturing problems that are holding up approval of a final design. Or perhaps the final design has been solidified, but there remain some tech problems in production. I recall reading here on MR that there had been some problems with the new "matte" displays.
I would think a mid-August release might have worked best for Apple from a marketing standpoint, as new models introduced then could be advertised with a "back-to-school" slant. Obviously, a release "into September" cuts into sales of the new model, as most students will have already made their purchases by that date...
I agree with you to cetain extend. The i believe there wont be a FULL overhall design change. They might just slim it down a bit to make look sexier, and im pretty sure they are gonna get rid of the super drive. as for mid august i hope so! but now im re thinking stuff and it seems more like October. And as far as a "matte" option; i dont think so the last thing we heard was screen lamination issues. However i do believe apple is going to use the same anti reflective glass that it uses on the Retina macbook pro. I dont think well see a complete "matte" screen.