So if there is a new entry every minute and they're reloading every 30 seconds, they'll only see new stuff every 6 reloads, not every 2, if their allow reload is 3 minutes.
Unfortunately, this doesn't save CPU, bandwidth, or DB I/O. I think we should instead say (to free users), on a white blank page "Whoa, reloading kinda fast there, aren't we? If you're this addicted, go get a paid account, otherwise reload in 7 minutes and 24 seconds" with a "503 Out of Resources" or something, so it's not cached/indexed on accident. The detection would be done on how quick logged-in users are reloading _any_ friends page, or if nobody's logged in, then the IP address, but IP addresses would have slightly higher limits, to minimize false positives from people coming through proxy servers. Or hell, maybe we go back to giving everybody (even not-logged-in users) a random cookie (ala mod_unique or whatever).
But the point is: people who are reloading every 2 minutes should be told they're being annoying, rather than appeasing them at the cost of site performance.
I'm sure many people disagree with me here, but I get angry and desperate when the site's slow. Could this at least be acceptable in bad times, like now? We could lower the threshold time when things are good.... bitching only when people reload, say, every 30 seconds or something.