The N900's built-in media player is a UPnP/DLNA client only (Yes, the N900 is without any surprise a "unofficially" certified DLNA device), to use the N900 as a sever too, you may install MediaBox Media Center for example.

The UPnP subsystem of Maemo5 is based on gupnp. it can streams Videos, both HTTP and RTSP streaming are supported with following content formats : 3GPP formats (H.263), Flash Video, ASF, .AVI, H.264/AVC, MP4, MPG, .MOV, .RA, .RAM, .RM, .RMVB and WMV.

In addition, HTTP streaming is also supported with : AAC, AAC+, eAAC+, M4A, MP3, WAV, WMA AMR, and MP2.

As i said, Nokia N900 is not officially DLNA or UPnP certified, but it includes features that enable to use the device with DLNA and UPnP enabled media servers.

Check this video out to see how it perfoms with Orb and XBMC :

The functionality is more user friendly than what is currently available from any Nokia or Samsung in date when streaming from source to phone...