Unloading content (ie external swfs) in AS3 is difficult, if not impossible.
My pal Martin Sieg at marblemedia threw me to an article by Grant Skinner – Failure to Unload: Flash Player 9’s Dirty Secret – where the author discusses a player bug that prevents content unloading. The problem impacts gallery-style projects, and sites that use embedded advertising.
Apparently there is no real solution, but Skinner is campaigning for a sandbox mode in Flash Player 10 where you can rope off your embedded content, and prevent it from getting its hooks into your main movie.
A noble effort, Grant. In other news, aid group World Vision announced today that rising food prices have created a shortfall that will prevent them from feeding millions of people.