UPDATE 12/19/09: Hulu will put up all the Season 5 episodes starting on January 1, 2010. Netflix has the whole season available now, for Netflix subscribers.
Original post below:
Until recently, they had all the episodes from Seasons 1 to 4, but only the last few episodes of Season 5. Now they’ve added the first 9 episodes of Season 5, but taken the later episodes down. I guess it must be some kind of marketing ploy, a way to get people to buy the Season 5 DVD set when it becomes available, but it’s awfully confusing.