I don't understand. How not constitutional?
The Senate is still in session someone said upthread.
According to the Constitution, the pocket veto can only be used when Congress is not in session. If Congress is in session, a bill becomes law automatically without the President's signature after 10 days, unless he explicitly vetoes it.
Obama hasn't explicitly vetoed it. He has just refused to sign it. The administration (via Gibbs) has announced this. But, the Senate is still in session. So, the bill should become law in about a week (I'm not sure when it was actually passed by the Senate).
You are probably thinking: what's the difference? A regular veto can be overridden by the House and Senate. A pocket veto cannot be overridden, because Congress is not in session. It permanently kills a bill, and a new bill must be filed and enacted by Congress when they resume.
This is an abuse of executive power. Once the precedent has been set, Obama (or a successor) can refuse to sign a bill and Congress would be powerless to override the pocket veto.