The way things are beginning to shape up, I would say the following:
May 2012 - updated iMac (silent update, no redesign)
June 2012 (WWDC) - OS X Mountain Lion announced, redesigned MacBook Pro released (or MacBook Pro/MacBook Air merge)
July/August - OS X Mountain Lion released
I would dare say even further:
iMac:
Ivy Bridge processors which will probably replace the Sandy Bridge ones found in the current iMac are out. The iMacs will possibly be equipped with Core i5-3470S (or i5-3550S), i5-3570S (with an option to i7-3770S) and i5-3570 (with an option to i7-3770).
The AMD 7000M series of video cards is also out, and these are supposed to equip the new iMacs. These video cards are not yet able to support a resolution of 5120x2880 (which would be HiDPI for the 2560x1440 screen of the 27" iMac), so probably we will not see retina iMacs this year.
With the processors and video cards already out, and almost a full year since the last iMac update, there is no reason why the iMac shouldn't get an update in May.
I don't expect the iMac to get a redesign this year. The iMac was redesigned in 2009 and no other all-in-one is capable of matching it yet. In addition, the MacBook Pro was redesigned in 2008 and will probably be redesigned again before the iMac. I would, however, expect a redesigned, retina iMac for 2013.
OS X Mountain Lion:
WWDC is going to happen in June. I expect OS X Mountain Lion to be announced with several features including HiDPI (Apple has not yet mentioned it, although it is present in the developer preview).
However, I expect Mountain Lion to be released in July/August. Apple announced it for late Summer, and June is not late Summer.
MacBook Pro:
Apple is going to need hardware with a retina display in order to showcase the HiDPI capabilities of Mountain Lion. A laptop capable of displaying high resolution graphics should be in the cards for this.
The MacBook Pro has not received a redesign in more than 3 years. It still looks great, but Apple could do better. So it may be time for a refresh.
Talks of a merge between the MacBook Pro and the MacBook Air have come up in the last few months. This is a possibility, but I would not count on it. Apple has traditionally kept two lines of notebooks and I am really curious whether it will scrap its tradition and merge the two of them or if it will keep them as separate products.
In either way, Apple is probably going to make the MacBook Pro thinner and lighter. Sony released a redesigned 13" Vaio Z series in 2011 with a 35W dual-core Core i7 processor and which is even lighter than a 11" MacBook Air. If Sony can do it, why can't Apple do any better?
A retina display shall come to this redesigned MacBook Pro. Apple has released a US$ 499 iPad with an IPS display capable of a much higher resolution than the top-of-line MacBook Pro. And the Galaxy Nexus, which is currently selling at Amazon.com for US$ 430 (unlocked) has a resolution of 1280x720, almost matching that of the US$ 1199 13" MacBook Pro. And the MacBook Pros still have a cheap TN display instead of IPS displays that equip the iPad, the iPhone and the iMac. Apple may be currently facing constraints in the supply of retina displays for the iPad, but this issue shall be solved soon with more suppliers joining Samsung. And, as iPads sell three times as much as Macs, the demand for MacBook Pros screen is much lower than for iPad screens. Supply should not be an issue. Seems that the time is up for an upgrade.
I would expect the retina MacBook Pro to be released during the WWDC. Apple will showcase Mountain Lion on its MacBook Pro, but will release it later. The retina MacBook Pros will ship with OS X Lion with HiDPI mode on (why else would Lion support it?) and will later receive the upgrade to Mountain Lion.
Doesn't it make sense?