Yesterday lunchtime I wrote a simple GStreamer app in python to run on the N900 with the video embedded in a widget inside the app. I committed it inside gst-python git inside the folder examples. It just does videotestsrc ! videoscale ! ximagesink. It could have been videotestsrc ! xvimagesink but I wanted to grab a screenshot.
Code (with comments): http://cgit.freedesktop.org/gstreamer/gst-python/tree/examples/maemogst.py