Executive privilegeCurious about Bush's signing statements? Here's a fabulous link that will fill you in.
For those who aren't aware of it, a Presidential Signing Statement is, essentially, an addendum a President attaches to a bill when he signs it into law, that basically says "Yes, I'm signing this damn thing, but I have certain reservations about it, which I will detail in this statement."
Other Presidents before Dubya have used signing statements -- it's a mechanism that isn't mentioned anywhere in the Constitution, but that past Presidents have used to express a lack of complete agreement with a bill that, for whatever reason, they don't hate enough to actually veto.
Dubya, however, loves signing statements to an unprecedented degree. He has issued hundreds of them. His primary purpose in doing this is two fold -- first, he doesn't want to risk having a veto overturned, which is generally something that costs an Administration an enormous amount of political capital. Second, he honestly doesn't see any reason why the laws Congress passes should be binding on him or anyone who works for him, and seems to truly believe that as Chief Executive, he has the power to rewrite and modify any and all laws to suit himself.
The link up there leads to a blog post that will show you the 'Top 10' signing statements Dubya has issued, but again, he's done this hundreds of times.