We just received word from PayPal today about this issue:
04/12/2011 08:10 AM
Thank you for bringing this issue to our attention. Unfortunately, we have been unsuccessful in recreating the issue. Please confirm that this problem is still occurring. If you are still seeing this issue, please send us additional information to help us identify or reproduce the issue.
PayPal Merchant Technical Services
Reading between the lines there, apparently the issue eventually vanished as mysteriously as it appeared, so all they can say is that it doesn't appear to be happening anymore. We haven't received any reports of this in recent memory, so let's hope that's the end of it.