Although this mail is 2 years old, did you happen to get an answer to your question regarding progressive jpeg?
Thanks for your answer,
I saw Teds post, he is talking about progresivaly downloading images but he doesn't use the special feature that 'progressive jpeg' has to offer. The issue here isn't streaming the image as a stream of bytes instead of a stream of image data but being able to decode the stream of 'progressive jpeg' image before the whole stream has been downloaded.
I saw a couiple of posts about this issue but they are quite old (2006) and I was wondering if in newer versions of Flex (Flash) 'progressive jpeg' is supported.