NEW YORK -- CBS' "The Late Show With David Letterman" tied its lowest-ever adults 18-49 rating last week. "Late Show" averaged a 0.9 rating in adults 18-49, Nielsen Media Research said. That is its lowest ever for an original since the program premiered in 1993, though not the lowest for the show: It has hit 0.9 several times, most recently in late December when it was in reruns because of the WGA strike. Meanwhile, "The Tonight Show With Jay Leno" averaged a 1.3 rating in the demo, its biggest margin of victory against "Late Show" in four months. "Late Show"...