[Splitting this post from an old thread]

I have this same issue using OSX 10.6.8 with the latest patches and MS Office 14.2.4 (Outlook 2011).  It seems that after pressing Preview, after 5-7 seconds, the attachment that's being previewed no longer is "highlighted" and that's when the preview window shows "No items selected".  It's as if another process is taking the focus away from that attachment to something else...