Apple's annual developer conference in San Francisco.
Leopard Still on Schedule for "Spring 2007" [Updated]
Again, "Spring 2007" gives us a timeframe up to and including June, which is when the Worldwide Developer's Conference takes place.
Meanwhile, there has also been recent speculation that Leopard is not far enough along to ship in April as (Page 2) rumored.
Update: Shaw Wu, an analyst, believes that "Mac OS X Leopard will ship on time in the 'Spring' timeframe, or Apple's June quarter" and that Apple is nearing final candidate release.