Well, it is official - "The Simpsons" has been renewed for a 23rd season, and will definitely reach 500 episodes (the 400th aired in May of 2007.)
It'll be quite the landmark: No primetime TV entertainment show on the air currently has reached 500 and the club for all shows happens to be very small - "Lassie" nearly reached 600 and "Gunsmoke" pushed past 600, so "The Simpsons" has a way to go before it reaches that mark.
When will the 500th air? Al Jean told Joe Adalian of "The Vulture" that he expects it to be in February of 2012. For the math, "The Simpsons" will have a total of 487 episodes at the end of this season, the 22nd.
The new order is for the 23rd, and accounting for preemptions - usually a couple in October and at least two, sometimes three, in December - the 500th could indeed air in February; it'll be the 13th - lucky number! - in the 23rd, which could place it in either February or March of '12.
Now: 600. I fully expect Fox will give "The Simpsons" an order for the 24th and 25th seasons. The 24th season will air over 2012-13; the 25th will air over 2013-14.
But here's the rub: By the end of the 25th season, 556 "Simpsons" will have aired. More simple math: To reach 600, Fox'll need to order a 26th and 27th season. The 26th will air over 2014-15; the 27th over 2015-16.
So remember that you read it here first: 1.) the 500th episode in Feb. of '12, but maybe March; 2.) Fox will certainly order a 24th and 25th - assuming it can come to terms with cast members and producers, and other related production entities, which is never a sure bet; 3.) If the foregoing actually happens, ratings remain solid (under 9 million viewers so far this season), and the international market for "The Simpsons" remains as strong as ever, then...the 600th will air sometime in 2016, probably April or possibly May.
Please mark your calenders.