The "USA PATRIOT" Act, like so many other acts of Congress over the years, is indeed unconstitutional. And it is technically true that attempting to enforce an unconstitutional law is an impeachable offense. The logic is as follows: 1. To say that a law is unconstitutional is simply to say that it doesn't exist, since it conflicts with law of a higher legal authority. 2. To attempt to enforce a law that doesn't exist, either through arrest of a person, invasion and seizure of property, or any other intrusive means, is to undertake those means without just cause. 3. If any private citizen were to do any of those things, it would be considered a felony, hence if an officer of government were to do so, it would certainly consitute grounds for impeachment.
If Bush is refraining from enforcing this particular law, then he's off the hook on this particular count. Of course, either way, I realize impeachment is not going to happen for something like this, but this is nonetheless the law as I'm able to see it.