I did in fact consider adding some hooks for payment gateways, but in the end I felt that it would be too specialized to do in a general way. A simple site can start easily and add a Google Checkout button (like I did) with a simple blob of HTML. Of course that's not a solution for everyone.

One possibility I thought of was adding support for payment verification callbacks. So you can create single-use digital downloads that are only authorized after payment is verified. If I ever get traction, that's definitely on my list for a future feature.

Thanks for the link to to the Saas starter kit- I'll take a look.