My Alpha/PNG background isn't working I downloaded a file for Blender that have the Sun animated, I only actually changed the animation parameters on the file, but nothing more than that.
Now i'm trying to render it with transparent background (not background at all) checking Transparent and then choosing PNG/RGBA, it works if i open a new file, but with this one, the alpha background isnt just working.

I uploaded the file (.blend) here, if someone finds a way to render it without any background (on this case, black):

http://www.mediafire.com/download/9kbom0wrwdxwpyp/sun.blend
You want to enable the 'Use Alpha' toggle in your output compositing nodes' properties (shown in their 'Properties' panel (N key) when they are selected).