Using Bedrock to create AJAX components is trivial. Name your Bedrock file with a .jroc extension and return your result.
<var --json $result>
Can it get any easier? I just created a wiki page that shows how Bedrock handles exceptions for application/json files you serve up using Bedrock. I also describe how you can customize the HTTP status code that Bedrock returns to the browser.
http://twiki.openbedrock.net/twiki/bin/view/Bedrock/BedrockStatusCodes