Using sample non square pixel tests from Microsoft site was inconclusive.
7 of the 27 sample encoded videos were 1:1 PAR. The others varied from 24:11 to 12:11
None of the videos played past the first frame, but all the videos was missing an audio track and I’m not sure if that had something to do with it.
Once again – the videos played normally under Windows Media player 11.
So this sheds a little more light on the problem – I know there is an issue, but the pattern is elusive.
James McParlane's Blog 
I think the audio track is key. Encoding the files from Adobe After Effects I can’t get them to work unless I encode an audio track. There doesn’t have to be audio in the file, but I have to set the encode options for audio anyway. Also files past 720p don’t seem to work.
Yep its a known issue that it won’t play without an audio track – I have been told that that will be resolved in RC1 – which is due out any second now…