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Okay, so yeah, from the documentation it looks like the Silent POST feature is only available to "Pro Hosted" which is the iFrame/hosted solution. I'm a bit surprised by it, but I simply am not finding any information about this feature being available to full PayFlow Pro users. We're going to need to get some news from one of the PayPal reps about this one.
Have you considered setting up a CRON job that pulls transaction data from your account on a given interval? It wouldn't be real-time like POST back would be, but you could run every few minutes if you need to and just pull anything new that you haven't pulled yet.
What you're running now is Payments Pro 2.0 + Recurring Billing, which uses the PayFlow gateway. If you were to setup Website Payments Pro 3.0 + Recurring Payments instead then you could use IPN the way you're wanting. There isn't any way to port these profiles from 2.0 to 3.0, though, so switching may not be an ideal solution for you unless you are very early in your process.
Also, don't get confused. 2.0 is actually newer than 3.0 believe it or not. For details on that you may be interested in my article on The History of PayPal Payments Pro.