Hello ther from a (very) old Opera Buff.
Having looked at the Sydney Opera House site I can see what you mean.
As their search box goes (at the present time) right up till July 2010,I think they have just not listed any performances past early December.As far as I can remember the Opera House only closes for Dec 25th,and maybe Jan 1st.
It's really a matter of keeping returning to the website to see how much more forward they have moved there calendar.
BTW,did you realise that what is now the Opera House was meant to be the Concert Hall and vice versa?
Well,Bjorn Utzon (the architect of the complete complex) resigned in disgust,because the government comittee charged with the building of the complex kept interfering and delaying finance etc.
Sadly,this committee were not theatre or opera people.They changed Utzon's plans,which meant that the proposed opera house only had 6 feet of wing space!
Not nearly enough for opera productions,so the two buildings were swapped as the concert hall had little wing space,but quite a bit more than the opera building.This resulted in millions of $Aus worth of high tech understage equpment being entombed in concrete,under the (now) Concert Hall stage!
There was/is only one good thing to come out of it.The acoustics in the (now) Opera House are better than if they had been in the previous building!
I was last there many years ago,when I saw Joan Sutherland in "The Merry Widow",one of her few forays into comedy roles:she was very good.
To finish,I think it's just a matter of going back to the website every couple of weeks,and seeing how far it has moved forward.
If you want any more info,please don't hesitate to post.