You can't use the "freedom of the press" to defame someone - there are reasonable limits.
Well, certainly slander is a limit.
But, that is it.
Anonymous sources are vital to journalism.
Without them, many corrupt govt. officials would never get revealed.
Does the press abuse them? Yes. But, that doesn't mean they should have their press freedom taken away.
Now, as for the Plame case, it will be interesting to see who the leaker is. I don't think this will be some horrible end-of-the-world scenario to have this source divulged in this relatively unimportant case.
But, it could stifle the ability in the future to get anonymos sources in stories that are vital and need them because people are afraid of retribution for talking.