I've noticed that MSNBC and TWC have aggressively expanded their use of Twitter in just the past month. At least two anchors and one TV show on MSNBC (as well as MSNBC.com feeds) have a Twitter, and on TWC, Heather, Jeff Mielcarz, Jim Cantore, A&B, and TWCi in general have one. TWCi and company follow a lot of TV station Twitters, including WOOD-TV, WXIA, and WCMH.
NBC News, The Today Show, Al Roker, Nightly News, Meet the Press, CNBC (inactive), and possibly others are also on Twitter.