Since David Letterman made an appearance in Conan O'Brien's guest chair, can we expect him to appear on CBS' The Late Late Show With Tom Snyder? ''I hope he will,'' says Snyder, whose new show is produced by Letterman's Worldwide Pants but without Dave's involvement. ''He's not going to get in the mix here. He said get it done, you know what you do.'' Letterman declined to comment. So while Snyder's waiting to hear, he's setting the tone for his . show. ''I want people to have a good time. I don't think folks want to see guests suffering on TV especially not at 1 a.m. Our competition isn't another talk show, it's Mr. Sandman.''