I've previously posted about Outlook's annoying Ctrl-Enter shortcut which instantly sends your email, even if you accidentally type Ctrl-Enter midway through composing your email. There are a number of workaround options including disabling the shortcut (see http://blogs.technet.com/usefultechnology/archive/2005/08/23/409572.aspx), adding a time delay to email sending (see http://blogs.technet.com/usefultechnology/archive/2006/01/20/417790.aspx) or simply religiously avoiding pressing the Ctrl-Enter key combination.
Help is at hand in the new version of Outlook which will be launched as part of the 2007 Microsoft Office System. In the new version (let's call it Outlook 2007) pressing Ctrl-Enter while composing an email will bring up a helpful warning dialog box. You can disable it by clicking the 'Do not show me again' option but my advice is to keep this warning enabled to help save your blushes from premature sending.