Hi, I don't use PayPal much now, mainly offline payments, but I still had a few PayPal accounts.
They have been running fine, with IPNs till August. And this August no IPNs have been processed.
Unfortunately this wasn't noticed until a site was disabled, so I have now set all the PayPals to permanent.
I have checked my paypal account and the ipn url is there (gives a '0' if sent with no parameters) (and of course so where the undetected payments)
Apart from whatever ProSite plugin updates the major thing I have done is upgraded to WP 3.6.
Whilst this doesn't impact me (anymore - as I rely on my accounting system to tell me about unpaid accounts) I think you need to check this out.