According to the official website, a new OVA will be distributed to those who purchase all the three Blu-ray box sets of Sayonara Zetsubou Sensei
series. It will be shipped in October. The total price of the three box sets is 69,720 yen ($850).
A live streaming of Sayonara Zetsubou Sensei will start on March 12th on Niconico Live. All the TV series will be aired without intermissions for 15 hours.
Source: Zetsubou Blu-ray official website
Sayonara Zetsubou Sensei Special