The next OS X Snow Leopard was previewed by Apple already almost a year ago at the WWDC 08. It is expected that Apple will launch Snow Leopard this summer. The Baltimore Sun (Via CrunchGear) predicts now June 8th as the release date of Snow Leopard. The reasoning behind this date is that the WWDC 09 could take place at the Moscone Center in SF from June 6th to June 12th.
Apple did not set the date yet for the WWDC 09, but the Moscone Center shows a corporate event for that time frame. Apple Insider also has a May timeframe that is also blocked as corporate event on the radar as WWDC 09 date in this report.

It of course makes sense for Apple to release OS X Snow Leopard in conjunction with the WWDC 09. As software releases are tricky to estimate, Apple will keep their options to set a date to the very last minute. Apple developer conferences have been also been much later in the year in the past.
More Apple News.