On the Zend Developer Zone, there's a helpful hint for users out there running a web service to help keep things safe:

Ok, so you've written a cool new Web API and you've written l33t JavaScript to call it from your page. The problem is, anybody who views your source can see how you call your new toy and use it for their own nefarious purposes. Granted, sometimes this is what you want but there are times when you want to keep your toys to yourself. For those times, here is a quick trick you can implement that will help thwart most evil doers.

The quick trick involves the placement of a secret variable into the session after the user has been validated. The sample code to make it work is provided and and example Ajax call illustrates how it's used. Then all the API has to do is check for that value in the data passed along.