One airline I know where you can earn online booking purchase miles on one programme and then credit the flights to another is Alaska.

On AS, you earn the bonus as soon as you purchase(!) and you can switch the flight(s) to whichever participating programme you want via ITM or web checkin.

I don't think this is possible on AA and US as you earn the bonus miles only when you fly.

[This message has been edited by terenz (edited 06-19-2003).]