If your store is configured to use the PayPal Website Payments Standard module for accepting payments (formerly also called "PayPal IPN" module), then this notice applies to you.

If your server has had to be specially configured to allow IPN traffic to/from your server in order to process payment notices, you'll need to have your hosting company open up the additional new IP address PayPal is adding effective April 7, 2011 https://ppmts.custhelp.com/app/answers/detail/a_id/92

This is a good time to remind you that you really should be using PayPal Express Checkout if you're accepting PayPal on your site. Using the old IPN/Website-Payments-Standard method for payments is becoming antiquated and has been known to have problems in various circumstances. Using the newer Express Checkout approach is far more reliable for completing payments and giving your customer up-to-date confirmation that their order has been paid and accepted by your store.
Naturally, also upgrading to the latest version of Zen Cart is similarly valuable and important.