Supported or not :-/ I may have found a way - here's what I did:

1. created a Force.com Site,

2. saved a Session.page containing the following:

   <apex:page contentType="text/plain">{!GETSESSIONID()}</apex:page>
   

3. edited the list of Site Visualforce Pages on the Site to include it,

4. viewed the page at the Secure Web Address (mysite.secure.force.com/Session)

This (apparently) yields the Site Guest User's current Session ID. Taking the string and placing it in the Authorization: OAuth ... header works when subscribing to a topic at /cometd/27.0.

And sure enough, it receives real notifications containing real data. Love this platform!

Supported or not :-/ I may have found a way - here's what I did:

1. created a Force.com Site,

2. saved a Session.page containing the following:

   <apex:page contentType="text/plain">{!GETSESSIONID()}</apex:page>
   

3. edited the list of Site Visualforce Pages on the Site to include it,

4. viewed the page at the Secure Web Address (mysite.secure.force.com/Session)

This (apparently) yields the Site Guest User's current Session ID. Taking the string and placing it in the Authorization: OAuth ... header works when subscribing to a topic at /cometd/27.0.

Edit: more testing needed...