Are you having troubles playing an H.264 file in flash? Do you have to download the whole file before it plays? Probably you experiencing this issue because the moov atom has been placed at the end of the file. Renaun Erickson wrote a small AIR app that moves the atom to the begin of the file so it can be played out as soon as it buffered enough data. http://renaun.com/air/QTIndexSwapper.air
If you using a streaming server (Flash Media Server 3) you actually shouldn’t experience this issue, as it doesn’t need to read the atom to be able to play out, at the end it’s streaming right? 
The H.264 and the moov atom by Tiago's Weblog, unless otherwise expressly stated, is licensed under a Creative Commons Attribution 2.5 Switzerland License.
Thank you for posting this, you saved a client.
Not great if your hunting for one that will run on linux
Hey,Tiago,Before reading your blog,I had met the problem like you said.
Ours encoding engineer had fix that by change the encode parameters.
But here comes a new problem, I found that ,the longer the mp4 video file is,the bigger header data it has,when I send a 1 hour length video to our project ,you must wait for a long time(about 10 or more seconds,to a flash video ,it’s long enough to make user fretful),untill the header been downloaded,then you can play it.
Have you ever met this situation? Do you have any suggestion?
waiting for your reply~~
Very cool. Saved me a lotta headache.