As mentioned in this article: http://msnbc.msn.com/id/14020234/?GT1=8307
Senate Jucidiary Committee Chairman Arlen Spectre is prepared to bring suit against the Bush Administration over the so-called "Signing Statements" Bush has applied to over 750 bills he has signed into law. These statements are expressions by the President of an understood prerogative of the President -- to NOT enforce certain laws. In US history many Presidents have decided that their oath to uphold the Constitution overruled the laws written by Congress. This audio segment explains the situation.

That is not to say that Bush is a keen Constitutionalist. Far from it. But if Congress does not approve of him not enforcing certain laws, they are suposed to IMPEACH him, not sue him!