Snow Leopard - Previewed in WWDC '08, announced in WWDC '09 (shipping info etc), released in August 2009
I somehow doubt that Apple can release 10.7 only 1.5 years after 10.6 since iOS has required a lot work lately. Of course depends what Apple adds there.
I'm agreeing with the late 2011.
ok so in all honesty there isn't much to look forward to in terms of the 20th. Sure they might announce a new OS but big deal when its still going to probably take about a year to get released, who knows what might still change. It was cool first time i heard Steve talk about the magic stuff thats going to come out but its becoming boring and old, especially when Apple doenst live up to the hype its creating. Almost every product from the last few years was (and not meaning to step on anyone's toes) one huge disappointment for me. Ipad no USB, Iphone needed a bumper to work correctly, macpro update was extremely disappointing in terms of hardware and pricing, Imac aka the yellow tinted box, etc...etc yet Steve was always talking about "how magical it all is"