Can Amazon Simple Notification Service “push” notifications to a desktop client? It this possible as a true push without polling?

No, this is not (yet) available out of the box – Amazon SNS indeed works by publishers sending messages to topics, which in turn push these to endpoints subscribed to this topic. The currently available protocols for the Subscribe API are:

• http — delivery of JSON-encoded message via HTTP POST
• https –delivery of JSON-encoded message via HTTPS POST
• email — delivery of message via SMTP
• email-json — delivery of JSON-encoded message via SMTP
• sms — delivery of message via SMS [currently limited to the US region(s)]
• sqs — delivery of JSON-encoded message to an Amazon SQS queue

So the closest you currently have for push notifications to a desktop client is email, though it should be relatively easy to build custom ones by posting HTTP to a respective web service, which in turn could facilitate WebSocket or Growl etc.

Good luck!