Congressional and presidential use of agents to represent their branches has met constitutional muster. As the Supreme Court said in 1938, "The Constitution does not define what shall constitute a return of a bill or deny the use of appropriate agencies in effecting the return."
The founders inserted the pocket veto in the Constitution to prevent Congress from passing a bill and adjourning to prevent an anticipated veto. But they made it conditional so it would not be abused by the president; they emphatically rejected the idea that the president should have an absolute, monarchical veto that could not be overridden.
In this case, Bush tried to have it both ways. He pocket vetoed the bill as if Congress were entirely out of session -- but then he did, in fact, return it to Congress by sending it and an outline of his objections to the House clerk. He did so, according to his veto message, "to leave no doubt that the bill is being vetoed."
Bush's dodgy veto gambit mimics similar action by his father, who claimed pocket vetoes of two bills that he also returned to Congress. In both instances, Congress rejected the president's claims and instead treated the bills as "returned." Neither was overridden.
In 2000, President Clinton tried the same thing three times, prompting leaders of both parties to object. All three were treated as return vetoes; these weren't overridden either.
If this all sounds like constitutional arcania, consider the outcome if Bush's faux pocket veto stands unchallenged: Presidents would have absolute veto power any time Congress is not actually in session, bestowing on the chief executive the very authority the founders sought to deny the office. And why did Bush use this veto gambit now? Maybe because the bill in question passed by veto-proof margins.
Regardless of the motive, the Constitution does not allow presidents to pick the kind of veto they wish to use, and it certainly does not condone a pocket veto just because an override is likely. The existing regular veto is plenty potent, and Congress cannot be denied its constitutional right to review vetoes as long as bill return is possible. Congress should do what it did before: treat Bush's action as a return veto because the bill was returned. And presidents should curb the impulse to play fast and loose with constitutional powers.