After upgrading to version 111206, s2vars were unpopulated, as recorded by the clickbank-ipn.log.
Transactions show complete/successful s2vars up until version 111206, and then they suddenly stop working. No other changes to the site or any plugins were made before, during, and shortly after the upgrade.
One of the noticeable effects of the missing variables is that registration emails are not sent out after purchase (but the purchase itself goes thru fine). The registration/password emails, however, are critical to successful transactions. So..
After triple-checking everything and pulling my remaining few hairs out, I had no other solution but to roll back to version 111105, and viola - everything is once again working fine.
Moral of the story: evidence suggests some sort of issue or bug with the latest version of s2Member Pro (111206) and empty s2vars.
Until this is resolved, I have no choice but to avoid updates and stick with 111105, which is the latest working version of Pro I have available.
I hope this information is useful. It would be great to upgrade to the latest and greatest, but I don't have time to fiddle with this right now. I'll check back on this thread later this week and see if any news..
Until then, Thanks for a great plugin and all your efforts