Kongregate, my favorite place for demonstrating the endless variety of Flash-based game potential (currently 8,334 free games), is now hosting a tutorial section designed to teach you how to build Flash-based games.
They provide a side-scrolling game called "Shoot!"...so you play the game, then you download the source code, and off you go. Currently they have three tutorials up with 6 more in the pipe.
I love this - how smart is Kongregate being?