Signing Statements vs. Line Item Vetos

Signing Statements

George Will (NYT) wrote an interesting OpEd piece on "Why the Line Item Veto (that Mitt Romney wants) is unconstitutional. If you would like to read it, check out the (free) New York Times web site for his Sunday Oct. 21st. piece
I subsequently sent him an email as follow:

Mr. Will,
Thank you for your, as usual, well written article. I did not realize that a "Line-item" veto could be contrary to the Constitution. Your article has made that clear.

Question:
Given that a bill must be approved as "it" is submitted to the President for his signature or veto, how is it that Bush (and others) can get away with "signing statements" that effectively change or even void the intent of the bill submitted?

The classic example that occurs to me is the McCain bill prohibiting torture. Bush's signing statement effectively renders this bill meaningless and unenforceable.

How is a signing statement different then from a line item veto?

Truly,
R. Paul Alderson