Assuming you'd prefer AA to UA miles, send AA an e-mail via the missing mileage request form on their Web site explaining the situation and stating what you want: miles for the trip as originally booked, since you upheld your side of the bargain by showing up on time ready to travel, but were involuntarily rerouted. I've gotten miles twice this way when my requests were reasonable. Feedback via these forums indicates that others have been as well.

You have to wait 15 days from the date of your flight to send in the form or it bounces back. In your case waiting to see if the miles show up is silly, but that's how it's programmed.

Asking for the UA miles is probably simpler if you don't want to go to the trouble.

Other folks on these boards indicate that they have successfully asked for both airlines' miles in similar situations. Some question the ethics of that. Others say it's only fair compensation for the rerouting hassle. I'm just the messenger.