
I have some working code for a 
Google Cloud Print Proxy.  It is currently written in PHP and uses the 
Zend framework.
I have written specific blog posts for each service endpoint, and to finish up I wanted to do a complete walk-through.
First I authenticate against a users Google Account with 
Google ClientLogin API.
Then using the 
Google Cloud Print Services Interface:
I can make calls to the following end points to manage printers:
I can make calls to the following end points to manage cloud print jobs:
You can receive print job notifications via persistent XMPP connection:
If you want to download the sample code for my Google Cloud Print Proxy work, you can download in the following formats:
kinlane-productions2
If you have any thoughts or ideas for an innovative Cloud Print Proxy, let me know.
 
UPDATE 2/28/2011 - I have finished the first prototype for the 
XMPP print job notification service.  This is critical piece to eliminate constant polling of /fetch service.