Handy when making a reservation. Make sure you have the proper adapter (AA uses a car cigarette lighter style plug, United uses a three prong EmPower connector). As of writing, American has power ports on all of their flights I've been on, you just need to select the right seats. United currently offers power on all of their flights while in First and Business Class (from what I've read), I seem to remember at least one flight having power in coach, but I don't remember when. Most of United's flights don't have power in coach. This is changing, I believe, but I don't know anything about the rollout schedule. Needless to say, none of this is official. And it's hard to get official information on anything related to this. I have no information on other airlines, since I don't fly on any other carriers regularly.
American Airlines Powerport Seat Maps
Note that the TWA planes recently acquired by American Airlines may not have power yet. IMHO, the best seats in the house are aisles 20 and 21 on MD-80's, which have both power ports and are exit rows. Quite the deal. The dots are power port locations. Don't choose the window seat, as the power ports are located beneath the aisle and middle seat in rows with three seats