I just released a new version of User Points which interfaces with Drupal's Ecommerce module.

It allows many forms of interaction between those two modules, including:

• Users can purchase points for real money. If the site admin sets up a non-shippable product, users can add it to their cart and pay for it as they do for a real item. A configurable multiplier factor allows one unit of currency to give the user more than one user point. This can be useful when used with certain other modules, for example my job search module, where employers pay to post jobs on a per node basis, or job seekers pay for posting resumes on a per resume basis.
• Users can use their points as a method for purchasing. They accumulate points by posting to the site, moderating, commenting, ...etc. and they they can use their hard earned points for other things. For example, they can redeem their points for additional roles on the site where they can access more content, or they can use it to pay for recurring subscriptions.
• Users who purchase merchandise or services from the site, can gain "reward points" for their purchase, and they can use that for future purchases. This is in addition to the psychological factor that we all fall for that we got something extra when we bought  the main thing.

Of course, users cannot use points to purchase points.

The above allows for some very creative community building usage, such as providing a web site specific currency (Republic Credits?), providing incentive for users to be more involved in the web site by posting, and moderating. 

I also updated a module I wrote a while ago, the COD (Cash on Delivery) module, to add a few more options (whet her to to mark the transaction as paid instantly, whether to mark the workflow as completed for non-shippable items, ...etc.)

I  have to compliment Matt Westgate for a well thought out API for the ecommerce suite.