If I want to politely refuse service on a web site due to temporary overload, the HTTP response 503 Service Unavailable seems appropriate. The spec mentions sending a Retry-after header with the 503.
Is there any point? Does Retry-after affect anything? Do browsers pay any attention to it?
As far as i'm aware, no browser pays attention to a Retry-after header. Proxies and caches might, but generally it's considered a bad idea to cache failures (errors). The only obvious benefit is, it seems Googlebot (and possibly other spiders) will pay attention to the header if it's there (and won't remove the page from the search engine's index for a while).
If it's trivial to add, go for it -- it never hurts. But i wouldn't recommend going out of your way to do it. It's only advisory anyway.