Its not used as a line item veto, its used in reference to how the bill is carried out, thats the intent.
However, by voicing concerns in a signing statement, and bringing up questions (as he is allowed to do), someone can sue to overturn the bill and cite the signing statements as part of their case...and its perfectly constitutional.
I don't see how there could be an issue for the courts merely on the basis of the statement. That would be advisory, there has to be an injured party for a case.
Well, if this plays out, hopefully we'll see if it's Constitutional or not. I'd be interested to see Clarence Thomas's view.