I have been running a WordPress site for a couple of years and twice I have stopped receiving Instant Payment Notifications (IPN) from PayPal. The first time was because my server was off for a few days and PayPal got bored and stopped.
But today PayPal simply changed my IPN settings to disabled for no reason at all. Re-enabling them restarted IPN.
PayPal is always changing its UI but you can find your IPN history and settings by viewing the site map as shown.
— update 2005-10-11–
The IPN was disabled because my IPN receiving code was crashing. I assume that if the receiving server is not present PayPal backs off but if it is erroring it disables the IPN service.