There is not a way to change the font to show a barcode unless you can generate a bar code. Here are two methods to consider:
1) If you are using Enterprise Version of SharePoint 2007, you might consider using the Records Management feature:
http://blogs.msdn.com/b/recman/archive/2006/08/10/694087.aspx
2) Although I have not tried this product, using javascript to generate the bar code might be a very good way to display barcodes. qListView supports a custom display option (see tab on far right of ezedit) where you can insert barcode generation javascript and a field from SharePoint (i.e., <%fieldname%> and qListForm allows you to create calculated fields that can use javascript to generate a form field value.
http://www.idautomation.com/javascript/UserManual.html#Barcode_integration_tutorial
Let me know if either of these approaches helps.
Curtis