As a quick hack on http://test.leuksman.com/ I've added a little notice that displays at the top of the page if it's been recently modified, reminding the user to check the edit history to see what's changed.
The color of the box and the approximate age gradually gets duller after about 10 minutes, 30 minutes, and 60 minutes, then finally disappears.
I believe this idea has been bounced around previously but I hadn't seen it implemented yet. Not sure if we want it, but something like this might be good: it can remind new visitors that things can change quickly, and not to worry quite so much about vandalism they may see.
The actual text is ugly and poorly formatted, but hey, it's a demo hack. ;)
There are two parts: SkinTemplate.php is modified to export a JavaScript variable containing the last modified date as a machine-readable Unix timestamp. Then a function in MediaWiki:Monobook.js can check the age and poke a message into the web page.
The advantage of using JavaScript for this is that the HTML output doesn't change as it ages, so it wouldn't require fancy cache expiration footwork. The disadvantage is that client-side clocks may be off, so there's a bit of a fudge factor. Also a small percentage of visitors will have JS off and won't see it, but it would be nonessential so that's not a big deal.
-- brion vibber (brion @ pobox.com)