very little. I have this posted on devforums.apple.com and noone has answered there either. I think technically we shouldn't be talking about this here as its under NDA. There is an iphone 3.0 forum there and I did find a post that is basically asking the same thing, but that developer is actually using 3.0 and is trying to duck the volume using iPodMusicPlayer and finding that it ducks his app's volume as well. They didn't say whether they had tried applictionMusicPlayer.
https://devforums.apple.com/thread/14473?tstart=0
this thread is in the 3.0 confidential forum