So when should you use REST, and when should you use SOAP/WSDL?
I discussed REST vs. SOAP/WSDL back in August 2005. Separately, I also discussed Ajax and Java. I've created two pages (on my wiki) to gather up the postings on those two threads: REST vs. SOAP-WSDL and Ajax and Java.
I don't know all the answers to when to use REST vs. WSDL, and judging from all the comments when I've brought this up before, different readers have very different opinions. However, there is one trend I'm noticing lately: REST and Ajax seem to go together well. I discuss this in the section "When to use what." This isn't the be all, end all answer, but hopefully it's a step in the right direction.