A new module has been released which allows you to output certain information from nBill in a Joomla (or Mambo) module. There are quite a few configuration options in the module parameters so that you can customise what is output according to your needs and how much space you have (eg. modules in the top or bottom positions generally have more horizontal space than modules in the left or right positions, so you can choose which columns are output accordingly).
This module is released under the GPL (as it uses the Joomla 1.5 API), and can be downloaded from this page:
http://www.nbill.co.uk/component/option,com_docman/Itemid,10/task,cat_view/gid,15/