Preview at WWDC likely in June, followed by September launch.
Apple to Update Nearly Entire Mac Lineup at WWDC?
The most solid candidate for an upgrade is the MacBook Pro, which has been reported by numerous sources to be gaining a slimmer form factor with the update. Apple's iMac and MacBook Air lines are also considered solid bets for upgrades next week. The report claims that both the MacBook Pro and MacBook Air will receive ultra-high resolution Retina displays, but while the iMac has also been rumored to be gaining the feature in the future, it is unclear whether it will make it into the next update of Apple's flagship desktop line.
If Apple is planning to only update four of its lines, the final spot would come down to the Mac mini and the Mac Pro, with 9to5Mac arguing that the Mac mini is the most likely to see an update given that just-released Ivy Bridge chips that could also be appearing in a 13-inch MacBook Pro would be suitable for a Mac mini refresh.
Xeon E5 chips suitable for a significant Mac Pro update have been available for several months now, but Apple has so far elected not to refresh its professional-level workstation product and there continue to be questions about its fate given that it has been nearly two years since it was last updated.
Finally, today's report indicates that Apple may also be taking the opportunity of a massive Mac update to also introduce new and updated accessories, although details on Apple's plans for those products remain unknown.