Hello,
Ferhan told me about this thread, and i'd wish to thank you guys for taking an interest in this.
I'm working in building a sort of pyramid scheme system. Users register to my client's system and pay a fee for the registration. In the case the user(let's call him Jimmy) was invited to the site by another user, this latter user (the parent - i'll call him Jim) will receive a commission of the registration fee. Commissions are generated together with the IPN script (so they are calculated on the AlertURL page) and saved in the system's commission database. So far so good. The problem becomes clear when the site administrator tries to pay commissions. Basically in this case, from AP's point of view, the admin is the client and the parent - Jim is the merchant, so the AlertURL for this payment is the one defined in the Jim's AP account. So i cannot use IPN in order to update the status of the commission, cause the IPN won't ever return to the system the payment data.
Apparently this is a minor inconvenient, but in general, any payments generated on a site for a given number of users/entities won't be able to use IPN.
Imho your system is great when it comes to receiving payments, simple, effective, easy to understand, but when it comes to issuing payments, it cannot be used at all. And in this case the complicated socket notification of the paypal comes in really handy.