I would love to see this too in Nginx.

The pattern is described here: http://www.mnot.net/blog/2007/12/12/stale

The only software I've seen this implemented in is Squid 2.7 (stale-while-revalidate).

From time to time I see backends taking 800ms+ to generate a page at low traffic. It would be nice to hide most of these requests to the end user.