Today may be the first day of spring, but in Hollywood, it's still pilot season. Here are the latest castings: Oscar-nominated Gosford Park actress Helen Mirren has landed a lead role in CBS's Washington D.C. drama Georgetown; former Beverly Hills, 90210 star Jennie Garth is joining the cast of an untitled WB comedy set in Manhattan; and ex-Murphy Brown regular Grant Shaud will play an unlucky dad in the Fox sitcom Oliver Beene. Finally, Billy Baldwin (the cute one) is poised to make his TV series debut in the CBS cop drama R.U.S.H.