A few things you didn't know about former teen dream David Cassidy. (1) He wrote the theme song to NBC's John Larroquette Show: ''I submitted the music under a false name, Blind Lemon Jackson. When the producers called they were surprised that they were talking to David Cassidy.'' (2) He lost most of his Partridge Family memorabilia in January's earthquake: ''I had the stuff in a big box that was crushed. I think even the lunch box is a goner.'' (3) He's superstitious: Cassidy refuses to talk about either of his TV pilots. ''I've had more things fall apart in the 11th hour than I care to remember,'' he says. Maybe he'll get another lunch box out of the deal.