I gather most js libraries will now provide an error handler for 4xx and 5xx, which is quite handy to know whether to display an error dialog. I also use 4xx for when client errors eg when a db constraint fails, I respond with a 4xx and put the error message in the body of the response. For 5xx I log the error message rather than include it in the response.
There is one YUI component however, yui-uploader, which doesn't seem to treat 4xx as an error on FF for Windows (but it did on a Mac)
A favourite status code? If only we could respond to our managers with status codes when they ask for unreasonable things or don't give us the full details, any of the 4xx codes would be my favourite ;)