If a off duty police officer if flying comercially, can he/she carry a gun on the plane? Concealed?

Pre- 9/11, yes, as a duly-sworn officer of the law, that was never a problem. Post - 9/11, with the government goons, I mean the TSA, shaking people down for toothpicks, etc., I don't know.

Some can. In California, an off duty CHP officer flying to things like court can, but from what I hear it is more trouble than it is worth...

In general, off duty police are not allowed to be armed on an airplane. A CHP officer flying to court is flying in a job related capicity and has state wide jurisdication, hence he is technically "on duty." I think there are very few circumstances when even an on duty officer is allowed to actually carry the weapon on the plane. It believe it can be put in check in luggage unloaded.

Off duty police are very limited where they can carry a weapon. Usually in their own state and jurisdiction. Very few states allow off duty officers from other states to carry a concealed weapon. There have been attempts at national legislation allowing this. A few states recognize each others concealed carry permits. The NRA has a lot of information regarding this on their web site.