The first comeback was actually two straight to DVD movies that aired, broken up into episodes (which was nowhere near a full season's worth). It was never announced as to whether or not the show would be picked up as a series again until March. In fact, when Comedy Central was talking about bringing back the show, there was question as to whether the voice actors would return. Now, we have it confirmed that it will be an actual season. It's in the can and ready to air this week.
One big issue I had with the movies was when they were aired, if you missed one, it was confusing.
