I don't understand the sense in silently updating a system. Where there's money to be made the best option is to promote, publicise and try and make the most money you can. The best option would be to release the MBA, get the spotlight and hype sales, then update the MBP in a month or two and get the spotlight and sales for the MBP.
People might say the MBP and MBA won't be competing for the same market but I don't believe that's true. In lots of threads I've seen someone say "Well the MacBook Pro's delayed... I might get the MacBook Air instead". It may make no sense to you, but to others what they want is a new machine. I doubt most people who buy pro's really 'need' them. They just want them, and there's no difference between wanting an MBA than a MBP.
apple wants all of the attention focused on the macbook air so that they can get higher profits. the people who actually care about the MBP will most likely be in the loop enough to know about whether or not it has been updated anyway