According to the Apple news and rumour site, the omnipresent and all-powerful "people familiar with the matter" say that between now and the gold master of Snow Leopard, upper-class Apple Developer Connection members will receive 3 "key builds", leading up to an August release. These will each deliver important changes, including the rumoured interface update.
Plain old Leopard will not be left out in the snow, however. Mac OS X 10.5.7 is on schedule for an April 2009 release, with ADC members receiving regular builds already. So far, it contains about 99 documented code corrections.