Hey there WPMU DEV,
I am looking at using the Pro Sites plugin soon, however I would really like to use the Mijireh Checkout as my payment solution. It allows the payment pages to be securely transferred to a SSL/PCI setup server while still keeping the site look the same. Please look into it. I would really like to use it because it means I don't need to worry about the security of the payments as they will all be processed off my server.
The site can be accessed at: http://www.mijireh.com
It uses the same payment process of its big brother Cart66 Cloud a full cart option for Wordpress. Site: http://cart66.com
However I don't know if this will be a suitable integration with Pro sites?
Either way, one of the two would be awesome!