In light of the popularity of his WebThumb site thumbnailing service, Joshua Eichorn is implementing a new feature to help reduce the number of requests to the server. The most popular request is made to see if a thumbnail is ready, and the new feature - a GET callback to the server making the request - will help to reduce them.
If you wondered an API that requires polling isn't a very good thing for scalability. On my current setup I can pretty easily handle about 20 status requests per second on top of my normal traffic, the problem is its not hard for a bad polling implmentation being run by one user to make that many requests.
To solve this problem im adding an addition to the Webthumb API that will allow you to skip polling all together. The basic idea is that your make an API request and when your thumbnail is complete i'll make a GET request back too your server telling you that the request is complete.
It's a simple matter of including a new "notify" tag in the XML message to the server with a URL to make the callback to. He even includes some sample PHP code to get you started with the new feature. The script handles the automatic download of the thumbnails when called from the WebThumb service.