Hi Chris,
Note that I released a small update recently for Site Membership Pro to fix a bug where free trial memberships were not working if using only PayPal or Google Checkout. ie the user was not getting the option to proceed with the free checkout.
Currently we don't have any automatic re-billing for membership renewals. I do plan to investigate some of the available recurring payment features of PayPal, World Pay etc for possible future enhancements to Site Membership Pro. But I have no ETA on that at this time, its just in the consideration phase right now.
But, what it can do today is automate reminder emails that prompt the user to renew the membership. You first create reminder email templates then you can configure as many reminders as you'd like on the Membership Product edit page. ie you can configure the reminder for given time periods before the current membership expires like say 1 month, 7 days, 1 day and you can taylor the email template used for each one to have more urgent calls to action. If the user renews after the first reminder he won't receive the later reminders, but if he does not then he'll get the next reminders until he does renew. In fact you can configure one for after the current membership expires so you can have an automatic email telling the user that his membership has expired and urging him to come back.
Hope that helps,
Joe