We've been asked a small handful of times. I've not seen one either. If you want to drop us an email, Michael, with the details on the features you'd like to see we could consider it. The biggest question that initially pops into my mind is how to handle the incremental payments with so many different payment systems out there. Might be why one has not been made yet.
You also get into PCI compliance issues if you store CC numbers. There are very strict rules about storage of CC numbers that most store owners either do not know or do not follow.
Thanks,
Carrie