RSS aggregators (such as Google Reader) make good use of this header resulting in less useless traffic on your servers.
The tag should be present on RSS feeds, along with Last-Modified.
UPDATE: Well, I added this to the /question pages for search bots. We now respect If-Modified-Since on question retrievals for bots, and we return Last-Modified for bots.
GET /questions/1696044/how-to-delete-a-row-in-a-sqlite-database-table HTTP/1.1
User-Agent: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
Expires: Sun, 08 Nov 2009 15:23:33 GMT
Last-Modified: Sun, 08 Nov 2009 15:03:18 GMT
Date: Sun, 08 Nov 2009 15:23:32 GMT
UPDATE 2: It's working! I see 304s in the log from Google. Which is a nice break from the 2+GB of data Google pulls down from us every day. You see, the Google giveth, and the Google taketh away...