I think the latest iMacs will come will be at WWDC, July is too far off. I think if the notebooks will be staggered in release due to i3 coming in June, iMacs will come first. OR soon after 15".
Next week wednesday am will have answers of some degree because ivy bridge will have been announced, apple had their financial call tuesday & tues/wed are usual update days.
Also, once wwdc invite comes, we can tell if it will be all software or not most likely