With the Ajax extension installed in 5.3, a page calling cfajaxproxy does not crash with a tag not supported error anymore, but it also doesn't render any of the JS. We have a call like so:
<cfajaxproxy cfc="inroll.model.service.ChildService" jsclassname="ChildService" />
In the current 5.3 snapshot, this doesn't happen. The tag is "working" in the sense that I can replace the path with an invalid CFC and it will error due to not finding the specified component - it just isn't creating the proxy.