IS a Mafia boss mentally unfit to stand trial just because he likes to wear his pajamas outdoors?

For his part, Vincent (Chin) Gigante, who is the boss in question, will be present in Brooklyn Federal Court for a hearing -- currently set for October -- before Judge Eugene Nickerson to determine an answer to that.

But the same can't be said with certainty of either his prosecutors or a key government witness.

The prosecutors, Charles Rose and Gregory O'Connell, have resigned from the Brooklyn U.S. attorney's office, and the the Justice Department hasn't made a final decision whether to appoint them as special prosecutors.

And Anthony (Gaspipe) Casso, the latest top mobster to turn government witness, may be held out of the case by whomever the feds end up using as prosecutors, according to law enforcement sources familiar with Casso's FBI debriefings.

These sources say Casso, who was expected to testify that Gigante is quite sane, notwithstanding his penchant for wearing nightclothes on long walks outdoors, may never take the witness stand after a slight misunderstanding with his federal keepers.

Reportedly, he held back information about two Luchese mobsters until FBI agents caught him at it - and threatened to void his cooperation deal.

The FBI and U.S. attorney's offices refused to talk about any of this yesterday.

As for Gigante: "No matter who the players are, we expect to proceed," says his lawyer, Barry Slotnick. "We believe nothing has changed since Mr. Gigante was found to be incompetent four years ago."