One thing I'm curious about is whether I need to aggregate theseSeems like something your database could answer. My "why don't they just" moment: query for all page updates, group by page, compute the interval between updates, histogram the results.
This is something I was thinking about earlier. There is a tension between the desire for timely notification and a desire not to be flooded. One possibility would be to generally aggregate them, but make an exception for an update when there has not been a prior update within that limited time period; in that case post the notification immediately.
So for example, if the wiki were to have a burst of updates the first notice would go out immediately, but subsequent ones would be gathered and report something like "n more updates in the last [time period]".
I don't really have a good answer for how long that time period should be. More than a minute, less than a day seems obvious to me. Perhaps one hour would be a reasonable compromise.