Even if prosecutors and judges refuse to uphold Congressional subpoenas sent to the White House, Congress can still put White House staffers in jail if they refuse to testify under oath before the House and Senate. I see no need to elaborate on this, as I believe the references I cite below tell the whole story:

The New Republic

House Arrest
by Jeffrey Rosen  
Post date 11.13.06 | Issue date 11.20.06

http://www.tnr.com/doc.mhtml?i=20061120& amp;s=rosen112006

Even if the Supreme Court eventually ruled against Congress in Conyers v. Gonzales, Congress could always enforce contempt citations on its own. In a little-used procedure, Congress has the power to punish recalcitrant witnesses for "inherent contempt." As Morton Rosenberg of the Congressional Research Service points out in an invaluable 1995 report on investigative oversight that House Democrats are now heavily consulting, the defiant witness can be brought before the House or Senate by the sergeant at arms, tried, and locked up in the capitol jail. (In 2004, a citizen-activist was sentenced to a six-month term there for "disrupting Congress" by demanding to testify at a judicial confirmation hearing.) This inherent contempt procedure hasn't been invoked by Congress for more than 70 years, because a cumbersome trial for contempt has the potential to grind Congress to a halt. But, if the White House is obdurate and the courts are unsympathetic, congressional Democrats might decide that a contempt trial--unlike a presidential impeachment--would be good politics as well as good theater. And, of course, the House is always free to impeach Gonzales for his refusal to cooperate, which might be less politically risky than an impeachment of Bush.

Venable LLP

Point of Order:
An Insider's Guide to Surviving Congressional
Investigations
By: Raymond Shepherd 1 and Don R. Berthiaume 2

http://www.venable.com/docs/pubs/1631.pd f