SQS is now in production.
The production release allows you to have an unlimited number of queues per account, with an unlimited number of items in each queue. Each item can be up to 256KB in length. There's a complete access control model for each queue -- you can independently control who is allowed to read and to write to each of your queues.
The SQS API is lean and mean, with straightforward APIs to let you create queues, send messages, receive messages, delete messages, list your queue collection, and to delete entire queues. If your queue has more than one reader, you can use the timed visibility model to ensure that each item will be processed, but that no item will be lost if a reader fails between the time that it receives and deletes a message.
Pricing is on a pay-as-you go basis, with no startup fee or minimum monthly charge. You pay 10 cents for every thousand messages sent ($0.0001 per message) and 20 cents per gigabyte of data transferred. You can learn more about pricing on the SQS Service Detail page.
We've got sample code in C#, Java and Perl, and we also have a Windows Communication Foundation (WCF) Add-in.