Client notification thru WCF

I am wondering if WCF can help me with ‘client’ notification requirement.  Our server has queues of work that needs to be processed by clients, but we would like to avoid having the clients poll the server.  I am looking for a way to ‘notify’ clients that work exists.  The client computers doing the processing range from personal computer, business computers behind corporate firewalls to traditional web servers.  Our queues are in the cloud (probably amazon SQS).  Looking for a solution that will scale to 100k - 500k users.

Does WCF provide any functionality that would help with this requirement?

The 2 key points (from my perspective)
- the client software applications logs in and declares that he is ready to work
- at some point in the future... the server needs to be able to contact the client to tell him work exists in the queue.

The communication part seem a lot like an IM client so I have looked at xmpp but hoping to find something even simpler than xmpp (if it exists)