Mark Pruett's talk on Ajax and Web Services
Mark argues that people go through an evolutionary path in the way they use Ajax, they start off sending back a chunk of HTML, they move on to sending back delimited text, then XML and finally some people move to JSON.
There a bunch of client side Ajax toolkits: prototype, Dojo, GWT (Google), Atlas (Microsoft), Rico, Zimbra, DWR. Some of these, like GWT and DWR are tightly coupled with the server side language. You probably want to pick a toolkit that doesn't do that...
Update: He's talking about the whole SOAP vs REST debate, and arguing that externally you should expose your services via REST, but internally SOAP might be more appropriate, or at least more frequently used. Which isn't necessarily the same thing.
Update: REST uses GET, PUT, POST and DELETE. Where GET will get data, a PUT will create a new resource, POST will update an existing resource and DELETE will delete an existing service. Although of course there are certain limitations to GET which means that you sometimes have to use POST anyway.
Update: He's just totally dismissed synchronous REST requests, arguing that you should always use asynchronous requests. Which seems like a lot of overhead for simpler services. Although he's also arguing that you shouldn't bother with XML, but you should be using JSON instead, which means you can do this,
my_json = eval ("(" + http_request.respinseText + ")");
which I think is probably officially evil.