I know that, on the surface, this seems to be a very attractive concept, especially to those of a more conservative persuasion. Republican or Democrat, the sheer amount of pork in each federal budget in my lifetime and longer has been riddled with these little bits of poor uses of taxpayer money. I believe that's a very serious issue and one that must be addressed, not only for ourselves, but for the generations to come. However, the Founders of our great nation enumerated the powers and responsibilities of each of the branches of our government, and more importantly, codified a clear system for each of the three branches of government to act as a check on the powers of each other. The line-item veto would be a clear violation of the Founders' intent and the Constitution of the United States of America, as it would hand some powers belonging to the Legislative branch to the Executive. In addition, as a purely practical matter, this only seems like a good idea when "your guy" is in power, whomever he, or eventually she, is or which party, if any, he or she belongs. What happens when "your guy" is out of office, and someone from the opposition party is in the White House? Also, who exactly do you think would be on the receiving end most often of the President's line-item veto power, should this become a part of our nation's laws? That, perhaps more than anything else, should give pause to Congressional proponents of this idea.
As a side-note, if a line-item veto were to exist, Senator Kerry's proposal would be nothing more than a placebo and would further weaken the separation of powers between the executive and legislative branches. I think I liked him better when he was lazy.