Just a quick update: I intend to release an Alpha version of nBill 2.0 early next week. It will not require a license key, but will cease to work on 31st December 2009. Only the back-end features will be available (front-end yet to be developed!). It will require PHP5 with mysqli, and will run on both Joomla 1.0 and Joomla 1.5 without legacy mode, as well as Mambo 4.6.
I managed to include a few things that I didn't think I would be able to, such as printable receipts, template-based e-mails, and better support for partial payments. More details to follow in a new announcement next week.
Update: Please see this post for information about nBill 2.0 Alpha:
http://www.nbill.co.uk/forum-smf/index.php/topic,1359.0.html