Webthumb now has an API. Its a simple REST API based on posting some simple XML.
It gives you the ability to request thumbnails, check on there status, and even download files.
To use the API register and then goto your user page, your API key is shown there.
Basic docs on the Webthumb API are available.
If you have any comments or suggestions leave them as comments.
The API is currently in beta so im sure there will be some changes and additions, but its a very usable state. If someone wraps the API for easy PHP access i’d be interested in hosting the code so if you write a wrapper please get in touch with me.