Hi Tom,
I've logged this in the project tracker. Actually I have plans for a lot of improvements to Web Invoice Pro, and things like automatic reminders and scheduling of recurring invoices are things I am looking into.
Right now I'm working on a new product Site Membership Pro and some of the things I need to do in that feature for reminders and renewals are very similar problems to solve so the solutions I work out for this product will be helpful for the next round of improvements for Web Invoice Pro.
Best,
Joe