Let's be logical about this: Condoleeza Rice has refused to testify under oath before the 9/11 commission.

The Bush administration claims that this is because presidential aides, like the National Security Advisor, do not testify before Congress.

This is blatantly untrue; Clinton National Security Advsior Sandy Berger testified before Congress under oath a few times. The principle to which Rice is appealing does not exist.

It must be, therefore, that Rice does not want to testify under oath because she has something to hide.
Something that the administration does not want the public to know. Something Rice could plausibly be asked about at the hearing.

What other explanation is there?

Update: After a week of negative publicity, Rice has given in. Call me cynical, but somehow I doubt the reason for the week-long refusal to testify was based on principle.