Like all touch based devices, the N900 also packs a proximity sensor to kill the touch screen sensitivity when you are making calls. But when you are not making calls, it is still running with no action assigned to it. Fortunately MohammadAG was upto the task and has created a shortcuts bar reminiscent of the Xpress Music devices like the Nokia 5800 which you can bring up by holding the proximity sensor for 1.2 seconds.
In addition to the shortcuts, a long press on the media player icon will bring up media controls. The app is available under the package name MediaBar and you can follow this thread for support and requests.