According to MacRumors, Taiwanese newspaper Apple Daily says the only reason the MacBook lines haven't already been updated is because of chip shortages from Intel. The same report claims that all MacBooks will be available with not only the newest Intel processors, but also up to 640 GB hard drives and 8-hour battery performance throughout the line.
The MacBook Pro line is long overdue for an update -- its last significant update was last June -- and I know of several people who have been holding off on a new purchase until the line is updated. With Apple the lone remaining major manufacturer yet to integrate Intel's newest i5 and i7 processors into their portables, the MacBook Pro has been a "wait to buy" prospect for at least three months now... and the people I've been talking to have been losing patience with the wait and eyeing up other manufacturers' devices. Hopefully this latest rumor bears some unibody aluminum fruit later this month, and I can tell my waiting friends and family members about the newest MacBook Pros instead of continuing to promise them that updates are coming "any day now" like I have been since January.
Apple MacBook Pro 15-inch with Retina Display (mid 2014)