I saw a yahoo link today or yesterday referring to curses, and one of the links was for the Seinfeld curse. Without clicking on the link, I suppose the curse is that the stars are all doomed to failing television shows as a result of their being so familiar as the characters in Seinfeld.
I don't think that's quite accurate, except maybe for Jerry himself. He's always going to be Jerry.
I think Jason Alexander and Julia Louise-Dreyfus are more victims of an Oscar-jinx type situation, where they're expected to star and carry any tripe they're put in, and usually that tripe is rancid (Listen Up, anyone?)
Michael Richards can't be a star, I don't think. He's got to play a sidekick/supporting actor role, which he did perfectly in Seinfeld. Much like Bull from Night Court. Therefore any vehicle that he's put in will fail because it will rely too much on schtick.
So it's not a curse for the most part, rather, it's self inflicted. If the former stars of Seinfeld exercised a little more discretion and chose parts slightly more diligently, they'd not suffer from post-Seinfeld doldrums.