. We will almost certainly see new Macs at WWDC which will ship with free upgrades to Mountain Lion which will come out mid-summer. I doubt we will see any new hardware upgrades before WWDC which is now only about five weeks away.
Both of which are unlikely.
First, Cook and Apple execs mentioned "late summer" for Mountain Lion in last quarterly results call. It is extremely unlikely Apple is going to move that up. [ Given the front page story about Filevault coughing up passwords in plain text in certain configurations, it would be almost beyond foolish for Apple to cut short they QA cycle on this release. Apple's test coverage on Lion was more than spotty. ]
Second, it doesn't really make sense to wait to release them all at WWDC. WWDC has more than enough issues to cover (Mountain Lion
and iOS6 ). Unless some specific Mac was critical for a ML feature demo there isn't good reason to wait. Since ML is coming late summer it makes sense to get several Mac out the door before the ML "blackout period" begins. Once 4-6 before ML's release date it doesn't really make sense to release 10.7.x delta release when the OS is on immediate verge of being superseded.
By June, the Quad based competitors of the iMac should be fully on the retail market and several weeks into online sales. There won't be a good reason to sit on those. Even less rational reason to squat on a Mac Pro update until June. [ unless falling backwards to the Xeon E3 which would be a strange move. ]
By early first week of June, they should be able to do a complete MBP revision. (unless Intel stumbles getting dual Ivy options out the door by June 4th). Again no real good pressing reason to wait.
Frankly, Apple tends not to do well when the stack up too much to do on a single fixed date. The FUBAR " MobileMe + iOS + iPhone" simultaneous launch several years back.
There is no big win and piling
all of the Mac models into one big dog and pony show. Incrementally releasing over several months gives them not only something new to talk about each month, but evens out unit/revenue numbers so don't have to bubbles in those numbers as much over the long term.