I have the same problem as others here, no IPN information returned but the order was completed at PayPal's site. I have been searching the forums and it seems to be affecting both the official PayPal IPN module and the one by DevOSC (3.15). It does not seem to affect everyone, but I think it must be on PayPal's side. It seems to have started within the last week because I was getting IPN's last week, but not this week(even in the Sandbox).
Sean