Yesterday, I asked someone via online chat at Authorize.net if they had changed anything on their end (security settings, etc.) that could explain the enrollment issue and they flatly said no. Granted the person may/may not know what they've changed or may not even think a change they made would impact enrollments.
I'm not saying it's not happening in other programs, but in the past couple of days I don't see any other online reports of Authorize.net enrollments/subscriptions that have stopped working for things like Wordpress/Buddypress, Joomla, etc.
Granted it happened Friday and maybe other reports will come in on Monday, but I would agree that whatever was changed at Authorize is only affecting Moodle installs because of some specific way in which Moodle is actually trying to process the verified enrollments.
On a slightly off topic note, does anyone have any updates on Authorize.net in 2.3? Before this recent problem appeared, I didn't know it wasn't included in core. I know there was an older plugin someone made available that seemed to allow things to still work, but it's very concerning Authorize.net is not built into core.
I feel for the person for whom this is their busiest weekend as I've only had to void 4-5 transactions so far, but for those that charge for individual courses, automating the payment/enrollment option is obviously a critical function for any LMS they use.
I understand PayPal can be used in Moodle 2+ and it may be the defacto/default form of payment in certain parts of the world and they have pending deals with McDonalds/Discover to expand their mobile presense, but there are a variety of valid reasons one might choose not to use PayPal as their default payment method.