David Zeiler of The Baltimore Sun points out that the event schedule for the Moscone Center in San Francisco is listing a "Corporate Meeting" running from June 6th through June 12th in Moscone West and speculates that this event is Apple's Worldwide Developers Conference (WWDC) 2009.
While Apple has historically varied the dates of WWDC from as early as May to as late as August, WWDC has been held on the second full work week of June each of the past two years. Several days ago, MacDailyNews pointed to a similar "Corporate Meeting" scheduled for May 16th through May 22nd and speculated that it may represent WWDC, but that event is no longer appearing on the Moscone Center's calendar.
Zeiler speculates that Mac OS X Snow Leopard will be released at WWDC. This would certainly not be a surprise, as Apple announced at last year's WWDC that Snow Leopard would ship in "about a year".
As additional evidence for a Snow Leopard release at that time, Zeiler points to several listings on Amazon.com for Snow Leopard-related books scheduled for release from mid-June through July. In our experience, however, neither Amazon nor publishers have any particular advance notice about software release dates, and the publication dates for their related books are merely based on "best guesses".