Several posters have explained why MS will not open-source Win7, citing cannibalisation of its revenue stream, revelation of vulnerabilities, loss of proprietary drivers and codecs etc. All true, and ultimately of course the loss of revenue is the main driver people focus on. MS has never not been obsessively greedy, always at its customers' expense.
But even if ways were found to replace the lost revenues (for example, those offering paid "Win7: The Clone" services of any kind pay a fat commission to MS), they still wouldn't do it. In fact, MS wouldn't open Win7 even if it was guaranteed a huge payday.
And that is not because folks will find some vulnerabilities, or rip off some of the code.
It's because (a) the quality of the coding will make MS look awful, and (b) some naked theft will be provable, probably right in the core of the system. MS, in short, knows that its reputation would be blasted to cinders if the knowledgeable world gets a good, sober look at the wretchedly inefficient, derivative, obscenely bloated rubbish that it spackled together to make its least awful operating system. El Reg would have a whole year's worth of dreadful headlines to publish, and run out of puns and goofy clichés inside a month.
And of course, the story would only get worse, because once out of Redmond's clutches Win7 would go from strength to strength. A grateful world would discover just how good, and nice to use is a well-supported, efficiently refactored Windows OS, designed for the productivity and ease of use of customers—rather than as a series of entrapments and wholesale spying.
MS can't open Win7 because it would choke to death on dirty laundry.