Well, it seems to be out early. After rickygervais.com and Ricky Gervais's (other) podcast made repeated references to the new season's starting on September 16, the new season seems to be available now.
Since the podcast was scheduled to start tomorrow, I thought I'd check Audible tonight to see if there was any advance info (Audible rarely puts up advance info).
Lo and behold, it was there and already available for purchase. Not as a podcast (i.e., four weeks with a new half hour each week), but as a two-hour file with the description, "all four episodes are here as a single download. And not only is it available now, it shows a release date of September 5, 2008.
So it's on my iPod now. I guess I know what I'll be listening to on the way to work tomorrow. More details after I've listened to some or all of it. [UPDATE 9/16: The promised "More details" can be found here.]
The description attached to the Audible file also says this about the stars:
"Ricky Gervais, of course, is the award-winning co-creator and star of The Office (upon which the current NBC hit series is based) and HBO's Extras. Steve Merchant, who also stars in Extras, is his long-time creative partner. Karl is a pile of protoplasm that mimics the functions of a human being."