"But, CBS got sloppy and posted the .pdf files. If they had simply run with the story, they would have never been found out."
CBS had to post them because the alleged memos had become of over-riding public interest since they were the very basis for the news report. There were no reasonable grounds on which copies could have been withheld. Other news mediums would have demanded copies. Finally, there was more than a little hubris by CBS. The result was that the forged memos were posted by CBS.
I agree with you that CBS would eventually have to release the documents, if other other news organizations pressured them into doing so. But, I don't think that would have happened. The Bush/Guard/AWOL story is old news. It would not have drawn much attention if the bloggers hadn't pushed the story.