Corecodec have confirmed on Twitter there will be a version of CorePlayer comin soon for the Nokia 5800XM. They had initially planned to have it out by June 2009, but looks like its been pushed out to July 2009.
If you don’t know what Coreplayer is, its like VLC Player for mobiles. Coreplayer will play near enough every audio & video file you can throw at it. I use it constantly on my E71 and N95 8GB for playing DivX films download from the net. No need to convert files, Coreplayer will play it.