According to the latest Pew report:

At national organizations (which includes print, TV and radio), the numbers break down like this: 34% liberal, 7% conservative. At local outlets: 23% liberal, 12% conservative. At Web sites: 27% call themselves liberals, 13% conservatives.

This contrasts with the self-assessment of the general public: 20% liberal, 33% conservative...

UPDATE: There are some quotes about the poll in 'Poll shows liberal tilt escalates in newsroom'. Also, it should be noted that asking someone whether they're "liberal" or "centrist" is a fairly meaningless question. Something like an Americanized version of the 2D political test - including more sociocultural questions - would be much better.