Paul, firstly thank you for developing the plugin. Perhaps this needs to be a feature request for wp-ecommerce but I would like to allow my users to be able to spend their karma points in the wp-ecommerce store.
Specifically, at checkout time I would like an input option to "use karma points" towards the purchase cost. I imagine an additional admin option which would attribute a £/pence value for each point.
Also, do you have plans for a karma points leader board widget, I think this would be a great addition to the plugin ?