Amazon recently announced "Simple Notification Service", here's basic information about SNS:
Amazon Simple Notification Service (Amazon SNS) is a web service that makes it easy to set up, operate, and send notifications from the cloud. It provides developers with a highly scalable, flexible, and cost-effective capability to publish messages from an application and immediately deliver them to subscribers or other applications. It is designed to make web-scale computing easier for developers.
Amazon SNS provides a simple web services interface that can be used to create topics you want to notify applications (or people) about, subscribe clients to these topics, publish messages, and have these messages delivered over clients’ protocol of choice (i.e. HTTP, email, etc.). Amazon SNS delivers notifications to clients using a “push” mechanism that eliminates the need to periodically check or “poll” for new information and updates. Amazon SNS can be leveraged to build highly reliable, event-driven workflows and messaging applications without the need for complex middleware and application management. The potential uses for Amazon SNS include monitoring applications, workflow systems, time-sensitive information updates, mobile applications, and many others. As with all Amazon Web Services, there are no up-front investments required, and you pay only for the resources you use.