The only Apple employees who really know are the managers and they keep their mouths shut. I sometimes think that leaking information would result in the employee being banished to working in a coal mine or sent to Gitmo.
Ok that's silly, but the execs have a lot at stake and know better than to blab.
Nothing will compare with the 'Unofficial "I'm waiting for Arrandale"' thread that ran for about a year until the Arrandale MBP release in April 2010. It was a very entertaining thread consisting of 7 parts for a total of about 37,000 posts and over a million reads. After the Arrandale CPUs were introduced at CES in January 2010 it really heated up with the weekly cycle of anticipation each Monday, followed by the Tuesday morning letdown posts.
I was mildly interested in a MacBook Pro, and was perusing all the forums. It was this series that got me hooked on MacRumors. I finally bought a 15" 2.8 GHz i7 MBP with the antiglare hi-res screen in Nov 2010.
My personal speculation/opinion is that there is a 90% chance that (at least) the 13" and 15" Ivy Bridge MBPs will be released between April and July 2012. Not the narrowest of windows, but not really that wide either in the grand scheme of things.
Quite possible there never was or will be a leak from Apple.
On the other hand, evidence suggests suppliers and others in the supply chain might not be so leak proof. See today's leak from 'Electricpig' which mentions the new MacBook Pro set to be released in April.