In a very timely fashion the guys at Elgg, along with the remarkable Mr Lamb, have just announced that “communities on Elgg.net all now have attached wikis.”
It’s only available at elgg.net at the moment but they’ll help you dig it into your elgg install and I’m sure it’ll come as standard before too long.
Good work fellas!
Update: Jeremy Smith writes that Blogs@Case have now been joined by Wiki@Case… and they most certainly have through a Mediawiki install seems like there’s a bit of a thing going on at the moment.