Earmarks are 100% unconstitutional. When RP submits an earmark - ANY earmark - he is 100% wrong.
Strangely, the Founders who wrote the Constitution didn't know that. They used earmarks. The habit was far less corrupt then, mostly involving exactly how much to spend on the construction of bridges and roads that linked the early states on vital trade goods routes.
If the president does not want to spend money on something that the Congress wants to spend money on, then even if the Congress has passed such a provision in their omnibus bill, the president can direct the relevant agency to do something else with the money. They can pass a provision, say to improve paving on a road in Yellowstone National Forest, have it pass unanimously in committee and unanimously in the House and in the Senate and in the conference committee and the president and the Dept of Transportation could still ignore it and just allocate that year's DepTrans budget however they want.
That is why we have to allow the custom of earmarks. So that Congress and especially the House's exclusive power of the purse remains with the people, not with the executive branch.
This last week, there is a new legal theory going around that somehow earmarks really are illegal and non-binding on the president. As closely as I can tell, this is much like the signing statements that Bush attaches to every major bill where he indicates how he reads a congressional bill submitted to him and how he intends to read it and enforce it and fulfill it. This is another attempt by the Bush faction to enlarge the power of the executive at the expense of Congress. If this keeps up, we may as well just call our president Caesar and sit around moaning about the loss of our republic.
While Congress has abused its earmark authority over the years, destroying the balance of power between the branches of government is no solution. The purse must stay in the House, where the Constitution says it belongs. It's not perfect, it's just the safest solution that the power of the purse is held only by the most democratic institution of the federal government and where changes can be made within two years by the voters if they don't like how they spend the taxes.
Let's not do violence to the Constitution just because there is some earmark abuse. There are times when earmarks are perfectly legitimate and are actually good for the country so the Congress can direct the money to specific targeted programs.