Recently ran into an issue when MS Outlook refused to open any .msg attachments. You’d click them and nothing. No error, no message, nothing. If you save the file to the Desktop and tried to open it, all that would happen would be that outlook would flash. Turns out it is a bug in Google Desktop [1]. If the Google desktop plug in was active in Outlook, it would fail to open the attachment. Disabling the plug in resolved the issue as did updating Google Desktop to the the latest rev. See the following link for more details.
[1] http://desktop.google.com/support/bin/answer.py?answer=58591