Historically WWDC is held in mid March but if the prediction by The Baltimore Sun is correct WWDC could be moving to a date in June. This speculation is fueled by the Moscone Center events schedule which has a corporate meeting listed between June 6th and 12th.
Although completely unconfirmed it would make perfect sense that Apple would be moving to a June date. Apple has set the product cycle of the iPhone for the one year intervals and the next version has been officially announced around June for the past two years.
While we are expecting the new iPhone to be announced in June already Snow Leopard might also be coming out at WWDC. Although Snow Leopard appeared to have a release date of Quarter 1 2009