Slightly off topic, but as someone who works at a company that uses Drupal for e-commerce sites, I'm surprised there isn't a good (free) module for recurring payments. Definitely sounds useful though - I'll have to bring it up sometime.
There were a handful of them but they've all been abandoned and are listed on the Drupal site as such, complete with the requisite 'our security update and notification policy doesn't apply to this' warnings.