According to Weekly Shounen Magazine vol.31, Koudansha announced the original anime DVD (OAD) of Sayonara Zetsubou Sensei "Goku Sayonara Zetsubou Sensei". These OADs are coming with the limited edition of volume 15 and 16 of the original manga to be released on Oct 17 2008 and Feb 17 2009, respectively.
DVD will be produced just as many as the number of the pre-orders.
Source: Scan of Weekly Shounen Magazine
Source: Official Web Page
Quote from the advertisement note of the manga volume 14
Why don't you aquire the casting vote for the realization of the third season?
Source: Koudansha Web page