Hi,
I've had the very same problem. I've set this in my php.ini:
default_charset =
default_mimetype =
(so I left those 2 both empty). As of version 4.0b4, php always outputs the content type headers, and they say to leave those two settings empty to get rid of it.
Strangly enough; that didn't work for me. I finally gave up and closed down for the day. When I restarted everything, voila the content headers where gone...
So make sure those are empty and all other settings are what they should be, then restart everything and it should work fine.
At least; it worked for me.