It was declared unconstitutional 20 years ago in Clinton v NYC. I actually agree with that decision even though I love the idea of a LIV. The constitution never gave the president the power to strike out "line items" from legislation passed by Congress.
The L.I.V SHOULD be something a Constitutional convention addresses, along with term limits and other things.
The line item veto does seem to be a violation of the separation of powers making the President a legislator through the power of omission.