I have just put out an update to the SaaS Rails Kit, a sample Rails
application that implements recurring billing. This update adds support
for
the Authorize.net payment gateway. With the Rails Kit and Authorize.net
CIM, you get a complete and very flexible subscription billing system
that
you can use as a starting point for building your software-as-a-service
Rails application, so you can skip the boring billing code and get
straight
to the fun code. 
More info on the update is here:
http://railskits.com/blog/2008/07/recurring-billing-with-authorizenet-cim/
–
Benjamin C.
http://railskits.com/ - Ready-made Rails code
http://catchthebest.com/ - Team-powered recruiting
http://www.bencurtis.com/ - Personal blog