Lorna Mitchell has a last minute suggestion of something to add to your web service that can really help out when developers using the API need it - a heartbeat.
The heartbeat shouldn't require any particular parameters or any authentication, since formatting data and passing credentials can be a stumbling block for those integrating with a service for the first time or those debugging issues. The heartbeat method can return some known data, perhaps an "I'm here" message, and maybe some version information.
She points out Flickr's flickr.test.echo call that does nothing but return whatever was sent to it. This kind of method can be very useful for applications that might need constant contact with the API or, inversely, need to know when the API isn't there and switch to an offline mode. A consistent heartbeat makes it easy to check these sort of things and to be sure that you get consistent feedback from your requests.