There has already been a great debate over this, on the Friends newsgroup. The only explaination that allows for Kramer not to be renting an apartment from a fictional character; is that in the Seinfeldverse (Seinfeld, MAY, Friends, The Single Guy, Pride & Joy, Mad Man of the People [the last two have been cancelled]) Mad About You is a sitcom about someone other then the Buckmans.
Stephen L.
Keeper of the NBC Cosmic Balance