I am glad I took con law this year, that will probably be a separation of powers question next year.
From a con law perspective its a bit dubious, but from another standpoint its good politics. And even if he didn't bother to write "signing statements" the executive branch would still have lots of influence on how the bill was adminstered. I think if you a normal person (and Bush is) and you have a point of view and you have the power to implement that point of view, and you can do so quietly without causing trouble for yourself, you are going to do that. I don't think the long term big picture consequences of this are that important, this is just the give and take of everyday politics.