I have tried to add comments in the viewer pane to some MP3s.
But what happens is that after saving the comments, the comments does not show up in the viewer pane for the MP3 files.
Apart from that, the MP3s become no longer playable by Windows Media Player 10.
I tried it here and didn't have any problems. The comment was still there if I closed the viewer and opened it again. The file also played okay in WMP11.
Maybe the problem is triggered by certain formats of existing tags. Could you find a small example file that the problem occurs for and send it to me in a private message?