Think about it: you have to build all the server-side logic anyway so why not it build it in such a way that it can be used equally well by a refreshing web page as by a call via XMLHttpRequest?
That was the way I went about adding the recent enhancements to The Session. I made sure that the server-side functions responsible for executing searches and returning results were abstracted enough so that they could be re-used easily by an Ajax script. If you wanted to follow this process of abstraction to its logical conclusion, then I guess you could have all data returned as XML. Then you could build your regular website, add a nice layer of Ajax enhancements and provide an API for web services as nice little bonus.
“East is East, and West is West, and never the twain shall meet”
To use this technology correctly, developers need to understand both worlds. The concept of progressive enhancement is probably a new one to server-side programmers while the idea of data abstraction may be new to client-side developers.
Which reminds me…
If PHP is your server-side language of choice, the future is looking quite rosy thanks to a helping hand from IBM:
“IBM is putting its corporate heft behind a popular open-source Web development technology called PHP, in a move meant to reach out to a broader set of developers.”