the background in AE is transparent, you just have to have the correct render settings.
As in the post a few down;
check your alpha channel by clicking on the little white square at the bottom of the comp window (next to the red/blue/green little square). You should see a black and white matte of your text shape. Good.
Now add to the render queue and choose a format that supports alpha channels. I’m not sure which formats Flash supports with alpha, but for video I choose Quicktime animation. Make sure that in the render settings RGB + Alpha is selected.
Michael Duff –
Southern Cross Ten, Australia