I use the BH-503, and all the controls seem to work fine - as long as the media player is open on the screen. If you are doing something else with the N900 topmost, I guess the commands from the headset do not get properly routed to the media player application.

So what I do is always leave the media player window open on the screen, then use the sideswitch to turn the screen off and stow away the N900. Now the media player obeys any command from the heasdset perfectly.

Should I be doing something else like browsing the web simultaneously, I would control the media player by first opening it on the screen and then using either the screen controls or the headset ones.