According to Wiki, "signing statements do not appear to have legal force by themselves. As a practical matter, they may give notice of the way that the Executive intends to implement a law, which may make them more significant than the text of the law itself." And, more scarily: "As of October 4, 2006, he [George W Bush] had signed 134 signing statements challenging 810 federal laws." There are currently no checks and balances but the American Bar Association has issued a report condemning some uses of signing statements, and Sen. Arlen Specter (interestingly, he is a Republican...) introduced a bill in July to instruct all state and federal courts to ignore presidential signing statements and instruct the Supreme Court to allow the U.S. Senate or U.S. House of Representatives to file suit in order to determine the constitutionality of signing statements. It'll be interesting to see what happens, especially with the new regime in both houses...