"On Friday, Sept. 3, the day after the convention ended, Mapes hit pay dirt. She told Howard her source had given her the documents."
It appears Kerry knew about the memos the day before CBS did. This is from RNC News Release
http://rnc.org/News/Read.aspx?ID=4731 ,
“We know that the Kerry campaign devised a strategy to attack the President’s National Guard service that began the same night the Republican National Convention ended, with Senator Kerry attacking the President’s guard service on the night of September 2."
“We know that the Kerry campaign devised a strategy to attack the President’s National Guard service that began the same night the Republican National Convention ended, with Senator Kerry attacking the President’s guard service on the night of September 2." from the LA Times
Emily Will, a professional document examiner in North Carolina consulted by the network to help assess two memos related to Bush's military service, said her copies showed a fax footer with a time stamp that read 6:41 p.m. Sept. 2.
I'm sure they knew what was coming before the memos were faxed.