REST makes use of the full range of HTTP response codes, so as to avoid
clients having to parse the returned entity body to try to figure out
what the server did with your request. This was all pretty new to me
(and some of my learning curve is still embedded in Wirebird;
fine-tuning the codes is on my to-do list), but at least I knew the
basics: 2xx means success, 3xx means something moved, 4xx means you did
something wrong, 5xx means the server did something wrong. 

Unfortunately, the current version of Mochikit (which I had just about
settled on) isn't quite so savvy: it apparently doesn't understand
success codes outside of 200. 

I'm not used to having to check for this sort of support; I'm glad
someone on the Mochi forum mentioned it before I got too committed.