According to TPM:

Daniel Marcus, a law professor at American University who served as general counsel for the Sept. 11 commission and was involved in the discussions about interviews with Al Qaeda leaders, said he had heard nothing about any tapes being destroyed.
If tapes were destroyed, he said, "it's a big deal, it's a very big deal," because it could amount to obstruction of justice to withhold evidence being sought in criminal or fact-finding investigations.

Harman, and therefore the Democratic Party, knew of the tapes in 2003, but decided not to inform the American people or do anything to expose the government's policy of torture. This knowledge was withheld from the American people throughout the Abu Ghraib scandal, which began in 2004. The tapes' existence was known by leading Democrats two years before the American people were first made aware that the US government had used water-boarding.

The AP goes on to report, "While key lawmakers were briefed on the CIA's intention to destroy the tapes, they were not notified two years later when the spy agency went through with the plan." It reports that Rockefeller "only learned of the tapes' destruction in November 2006."

Even if one were to accept this account as true, it means that the Democrats have known for over a year that these tapes were destroyed but decided to say nothing about it.

In September 2006, Rockefeller voted, along with 11 other Democrats in the Senate, for the Military Commissions Act. Both that act and the Detainee Treatment Act, passed in December 2005, included provisions shielding CIA operatives and Bush administration officials from prosecution for torture and other war crimes.

There is no "memory" left in media. It is the sound bite of the second, and nothing more.

Just try to get anybody to connect the dots between the Guamgate (Abramoff, Delay, Bush, Rove) and NSAgate (Gonzales and Bush), and USAgate (Rove, Gonzales and Bush). Obstruction of justice in every case.

Its not that significant. Its standard operating procedure.

  It seems unlikely that the CIA would be using analog videotape at this point in time, and if it was a digital medium, which is much more easily copied, then I suspect there are still copies in existence somewhere.  I have no doubt that the CIA made multiple copies, and once something is in digital format, the ease and speed with which it can be copied and stored usually means that it's very hard to get rid of completely.

We all know what that reason is.

Even the dumbest people among us, the 24 percenters who, through wilfull suspension of their own critical faculties and indifference to the truth, still support George Bush, know what that reason is.

Harman claims to have written a letter to CIA urging against destruction of the tapes.  

CIA, apparently, didn't tell them of actually destroying the tapes until some months after it happened.  This, despite having expressed the intent to do so about 2 years prior to actually destroying them. (NB:  taking 2 years - or even 2 weeks - between expressing an intent to do something wrong, being warned against it, and then doing it, puts big, red stamps of "GUILTY KNOWLEDGE" and "INTENTIONAL CONDUCT" all over the CIA's actions here.

I surmise, from the story as presented, that these two (and their Republican counterparts on the respective intel committees) were briefed on the existence of the tapes as a part of the "oversight" responsibilities of the respective committees.

The linked article expresses some outrage that Harman and Rockefeller did not go public with their knowledge either (A) when they found out about the tapes' existence, (B) when CIA expressed their intent to destroy them, or (C) when they found out about the actual destruction.  I'm not going to be so harsh on their non-disclosure, though I would have been tempted to make a disclosure were I in their shoes.  The fundamental problem these two faced was that to do so would have violated their security oaths and likely driven the CIA and other intel services to avoid oversight and not disclose what they were doing.  It's a close question, but I understand how they resolved it.  Moreover, it needs be remembered they were both in the minority at the times of all three events, and I'm sure that had some effect on the calculus.

Washington - The Justice Department and the Central Intelligence Agency's internal watchdog on Saturday began a joint preliminary inquiry into the spy agency's destruction of hundreds of hours of videotapes showing interrogations of top operatives of Al Qaeda.