I'm very satisfied with using Satollo.net's Newsletter Pro plugin for my mailing lists. And I'm using WP e-Commerce for my shop...
So I'm thinking: let's combine the two.
What I want to do isn't all that complicated. I've added a custom form field to my check out page (http://tambourhinoceros.net/home/shop) with a checkbox asking the customer to subscribe to the newsletter.
I would like to automatically add the email addresses who subscribe via the shop to the Newsletter Pro database of subscribers.