I assume this still works in SharePoint 2010: you should create a modified template file AlertTemplates_edited.xml in ...\14\TEMPLATE\XML (don't modify the original file - make a new version with a new name). You then need to use an stsadm command: stsadm -o updatealerttemplatefiles, and probably do an iisreset and bounce the timer service.
Another way, which may be preferable, is to disable the email alerts and create your own re-usable workflow instead in SharePoint Designer, using the Send Email action. This will give you the opportunity to define the email message format using SharePoint Designer. A bit more effort, but you don't need to touch the files on the server.