Usually Reliable Sources Say...
By searching on the string "Gates Security Memo" in your favorite search engine, you can get a pretty good sense of where and how the rumor grinds loudly into the public ear. Rather than detailing the precise sources of information involved except where they're too juicy to avoid mentioning by name I'd like to present a more abstract analysis of where and how the rumor mill works.
Numerous kinds of information outlets all had some coverage of the Gates leak within 36 hours of its release. If he hadn't sent his memo relatively late in the day (2:22 PM Pacific standard time follows the cutoff times for many daily news organizations, who seek to disperse their information by noon Eastern time), I'm guessing the lag time would've been 24 hours instead of the more typical 36 hours observed for many reports on the memo. Here's a rough breakdown of the kinds of organizations that reported on the memo:
Of the 7,000 or so hits that occurred for the search in several search engines (Yahoo!, AltaVista, Google, AskJeeves, etc.), about 4,000 or so were actually relevant (others were bogus hits reflecting partial matches or some other leaked Gates memo interestingly enough, this also showed me that all of his memos of this kind have become public).
About 1,500 or so were general news sources, varying from global/international publications from The Economist to The New York Times, to local newspapers or media outlets (such as radio and TV stations). This indicates that companies with large mind and marketshare a category to which Microsoft indisputably belongs will get broad coverage.
Another 500 or so were general computer trade publications across a variety of languages, ranging from ComputerWorld to Information Week to Wired magazine to numerous other publications of that ilk.
Over 2,000 fell into a variety of computer niches that would have reason to take special note of such news be it general Microsoft, Windows followers, or security sites and publications. To me, this is a clear indication that "tightly targeted" information resources are obligated to report on rumors as well as news that falls into their chosen target areas.
The remaining 500 resources can only be categorized as "miscellaneous," including various newsgroups, mailing lists, and archived chat information where the rumor (or breaking news, as it was so soon viewed) quickly became a topic of ardent discussion.
Interestingly, at least 50% of the elements in any category (except for the entirely bogus hits) were reflections of one another that is, repetitions of the same story in some other venue or format. To me, the most interesting reports were those that reproduced Gates' memo verbatim (as in the Information Week item mentioned earlier in this story).