CoinDesk's API has been published (they got back to me with a link).

The api is listed at http://www.coindesk.com/api/

I was able to set mine up with Exchange URL:
http://api.coindesk.com/v1/bpi/currentprice/USD.json

and JSON key:
rate

You can also specify other currencies in place of USD above. 
It seems to work fairly well and I'm glad to have the option to display a composite bitcoin value without the need to choose an exchange.