You wrote, "When will they learn to stop and verify the integrity of their sources and proof of what they write?"
The answer, of course, is: NEVER - as long as they can get away with it. With the mainstream media secure in their role as anti-Bush (some might say anti-American) cheerleaders and chief propaganda organ for the Democrat Party, the truth is merely an inconvenience standing in the way of a rousingly good partisan attack, IMO.
Iraq is a good case study of this. Since the MSM long ago determined that Iraq = Vietnam, a "story" that discredits American involvement and butresses their belief that Iraq is a lost cause fits the template of their bias and deserves maximum dissemination, and the facts (which don't need to be verified as they see it) be damned.
Another great example of this was the Dan Rather National Guard "memos" fiasco. When the "memos" were conclusively proven to be forgeries (and not very good ones at that), Rather was quietly allowed to retire without further consequence. SeeBS did manage to throw Mary Mapes (who quite delusionally still believes in the "memos'" authenticity) under the bus, as there was a need for someone to be sacrificed.
And the MSM wonders why their approval in the marketplace, as evidenced by newspaper circulation and revenue figures or TV "news" ratings, continues to plummet?