Here's another reason for William Petersen to grumble. The ''CSI: Crime Scene Investigation'' star has complained that CBS has diluted the franchise with spinoffs, even hinting that he'll quit when his contract is up in a couple years. Now, according to Variety and The Hollywood Reporter, CBS is bringing to Las Vegas another handsome scientist who'll probe people's innards. It's Rob Lowe, rebounding from his short-lived NBC drama ''The Lyon's Den'' to star in a new CBS pilot called ''Dr. Vegas.'' He'll play Dr. George Rivers, the staff medic at a casino. It may look like a comedown from serving in the White House on ''The West Wing,'' but at least in ''Vegas,'' he'll be working at a place where the house always wins.