It's bizarre finding this on Science Blog:

An article published in the latest issue of Presidential Studies Quarterly discusses how the Bush administration used a little-known policy tool of attaching presidential signing statements to bills to, in essence, veto them. The statements serve to increase presidential authority while rejecting that of Congress. Presidential signing statements alter specific provisions of legislation without sending bills back for possible veto overrides. This is as effective and substantive as a line-item veto and can nullify a range of statutory provisions even as a president signs the legislation that contained them into law. I had no idea this was possible.