D.C. just requires "one part notification," so a recording would have been legal, if both men were in Washington. I presume that a legal recording would be admissible.
http://www.callcorder.com/phone-recording-law-america.htm#State%20Laws%20(Table)
If that's true...and Tim Russert recorded the conversation between he and Libby- Russert's days as anyone's go-to guy by politicians in D.C. are finished.
It's hard to believe- but I guess it's possible.