I just heard this briefly on the news. First of all, Jerry IS NOT going to do the "mock documentary" that he first planned and he will extend the final show from a normal 30 minutes to a full hour. Seinfeld also said that he and Larry David, who WILL co-write the final show with him, have the story "pretty much worked out" and he expects the final ten episodes to be as good as "the best they've done". Another interesting thing is that the cast will tape the final episode in front of an audience, but when it comes time for "the final scene", the audience will have to leave and that he wants to keep "the not-so-bitter-ending scene" a surprise, but it will "leave you laughing"
One other note that the newscasters said was that there is a buzz about Seinfeld challenging not only Cheers single episode record, but challenging M*A*S*H's record for the most watched final episode/single episode of all time.