It occured to me that polling Recent Changes, based on the time since last polling is maybe another solution to this.
What I like about this is that, if it was implemented:
- It would automatically work for *any* MediaWiki installation - It would not be hard to add support for other wiki implementations - It's entirely external to MediaWiki (no code changes) and should be fairly low-impact (poll once per hour?)
You are then free to offer a variety of services to an unlimited number of users
- email on each edit - email no more than once per hour/day/week/month - email after 5 (or more) edits - sms / text to mobile/cell phone [1] - IM / irc of any variety - update rss feed
[1] In the UK you can do reverse billing, which means WP could earn money when WP sends an SMS to someone, assuming the someone has signed up for this service. I suggest this would be a way to raise revenues for WP and could be run as an entirely independent project.