CBS Corp., which depends more than any other media conglomerate on advertising for its lifeblood, is planning to expand one of its units that does not. Bloomberg News reported today (Monday) that CBS's Showtime pay-TV channel is currently developing a video service that would be accessible only to the channel's 18 million subscribers. A Showtime spokesperson declined to discuss the matter, but Bloomberg said that it was likely that the service would be similar to one being tested by Time Warner's HBO that will make all of the channel's programs available on demand via the Internet to subscribers.