9 to 5 Mac points out that the Moscone convention centre website has a "Corporate Event" scheduled for the last week of June in 2010.
The words "Corporate Event" are synonymous with Apple events as the company does not wish to reveal when WWDC will be held but Moscone requires to mark the week off on its calendar. In the last few years spotting these words on Moscone's calendar has been an accurate way of pinpointing the WWDC event which Apple doesn't official announce until March or April.
But going by historic dates the ones pinpointed are later than usual with Apple usually holding WWDC in early to mid June. Not that the Moscone is available for those dates.