It is something flash does as part of its compression of .swf files. If you have a large image with an alpha channel that is loaded dynamically, you are looking at loading a png-24 unless you manually embed each image in its own swf file and compile it with compression. The alternative is to use a tool like png2swf, part of the swfTools kit at:
pdf2swf is also very handy and can make logitically impractical projects into a quick conversion
Update, having used this in my last project quite a bit, I noticed the alpha channel is clipped for anything below about 25% alpha. Not really acceptable if you are dealing with semi-transparent areas!