So I've done some further research; and it does appear to be a valid PayPal request. Here is a link (on the Austrailian version of PayPal's website), explaining the technical details. At this point, I can only assume the changes will also be applied to US PayPal customers.
Quote from link above:
"The ipnpb.paypal.com and ipnpb.sandbox.paypal.com endpoints only accept HTTPS connections. If you currently use www.paypal.com, you should move to ipnpb.paypal.com when you update your code to use HTTPS."
It appears to specifically have to do with the url being used to postback to PayPal's IPN. This is the connection from YOUR site to PayPal; not the other way around. So when a sale is made on your site; and you send the data to PayPal; you should be using an HTTPS connection.
I run an eStore on WordPress; and use a plugin to facilitate sales. I have reached out to the plugin developer (coincidentally residing in Australia), to see if he can provide me with more details. I will post back here as new information develops.
