Observed in Firefox 2.0.0.14 and Internet Explorer 7.
Upon selecting a short HTML message in the messages list, the preview pane becomes blank and a Loading notification drops in at the top of the browser window. Then, however, nothing happens and Roundcube remains stuck in that state.
Rolling back to a previous SVN version seems to undo this problem suggesting that the change is fairly recent. Possibly in the MIME parser?