From PayPal:

We’ll be upgrading our system to improve the quality and reliability of our services, and as a result, PayPal will experience a series of limited service interruptions in the next few months. Although you may have received a system email, I wanted to ensure you received advance notice directly from me.

We’ll be performing system maintenance on May 26, 2011 from 11:04 PM PDT to approximately 11:30 PM PDT. The PayPal website and PayPal APIs will be offline during that time and buyers attempting to make payments will receive error messages and won’t be able to complete their purchases. Other services that will be down during this time include Website Payments Pro Payflow Edition and Batch Processing (batch.paypal.com). We’ll be sure to let you know the timing of future planned outages.

We’ve tried to choose a time that will minimize impact to your business and your customers. If you experience issues outside of 11:04 PM PDT – 11:30 PM PDT, you can file a ticket via www.PayPal.com/mts and copy me in the “mail copy to” field so I can help facilitate resolution.

There’s information about this maintenance event here, and you can monitor system updates on the PayPal Developer Network any time.

PayPal is committed to providing our customers a safer and more convenient way to pay and get paid online and we appreciate your patience as we improve our services. We apologize in advance for any inconvenience as a result of this brief interruption. If you have any questions, please don’t hesitate to contact me.