This thread was very useful. Thank you to everyone that has contributed. We also were breached and I found about 7 new admin accounts from the past three weeks but only three of them had bothered to do anything. I had several new plugins and some Base64 encoded PHP tied to the subscriptions.php. I tried to decode the php but it is a file within a file, within a file and my day is only so long. I haven't seen others mentioning this. Has anyone seen this or can speculate on why this php file would be targeted?
UPDATE: after 10 rounds of decode we found a hacker tool called c99madshell.php was what the plugin was. A description of what it does is here:
http://www.derekfountain.org/security_c99madshell.php
We are digging deeper into what may have been accessed in the DB.