The 2005-06 TV season doesn't technically begin until today, but already CBS is off to a promising start. Survivor began its 11th edition Thursday down 18 percent from a year ago, but still ruling its time slot with 17.8 million viewers between the ages of 18 and 49. More impressive, the alien-invasion drama Threshold averaged an audience of 8.5 million in the key demo and overnight made Carla Gugino cybergeeks' favorite screen saver.