I'm going to post an additional entry about this when time allows. db_writer
pointed out the distinction between Congressional investigations and criminal investigations--which was an argument I expected, but when he wrongly claimed that Congressional subpoenas have no force behind them I realized that I forgot to mention Contempt of Congress and the fact that obstructing a Congressional investigation is itself a federal crime. And the fact that Bush's executive order is trying to unilateraly overturn nearly 200 years of court rulings. And so on.
But I'll write more about that as soon as I can.