PayPal has updated their system status post to announce they are now implementing a fix for this issue. Within the hour, IPNs for new payments should be getting sent to us almost immediately (as normal), and they'll be gradually catching up with their backlog of delayed IPNs over the next 24 hours:
UPDATE 5: We have implemented a fix and IPNs for transactions processed after Jan 28, 10:47 AM PST (Jan 28, 6:47 PM GMT) are being sent now. We are expecting IPNs for new transactions to start being delivered in near real-time within the next hour. We are also estimating that missing IPNs for transactions processed between Jan 27, 11:04 PM PST (Jan 28, 7:04 AM GMT) and Jan 28, 10:47 AM PST (Jan 28, 6:47 PM GMT) will be delivered within the next 24 hours.
We will provide the next update by Jan 28, 3:30 PM PST (Jan 28, 11:30 PM GMT).
Sent Jan 28, 1:11 PM PST by SC
If you are selling downloads, you can use "Send free download link" in your Seller Admin to fulfill delayed orders manually, at least for impatient buyers who complain they don't want to wait for PayPal to catch up with sending us IPN for their payment. If you do this, you may also want to enter the PayPal Transaction ID for those orders in your "Block buyers/transactions" screen, so our system won't process the original order automatically once PayPal finally sends us the delayed IPN confirming payment for that order.
Thanks to everyone for your patience and understanding as we've all been waiting on PayPal to sort out the issue at their end. Suffice to say, this has been a most harrowing day for everyone involved on all sides, but it appears an end to the trouble is now clearly in sight.