I think a forum for each of the major apps, or perhaps just significant categories (web, mapping, IM, VOIP) would be great.

I don't think the Wiki engine matters too much. It's good to reduce the barrier to entry by having one set of credentials on both the forum and wiki, but I think you've already done that.

My only problem with the Wiki on this site is that it doesn't appear to be updated much. This happens a lot with other projects I've been active on: I think Wikis should be used for HowTos, but inevitably the information ends up in forum threads instead. I think that all can be done is to encourage the Wiki use with links in a top level sticky and then by example. e.g., a sticky at the top of each saying something like:

Subject: Read Me First
This is the forum for discussing X applications. Information and HowTos about X apps are here:

• app1
• app2
• app3

Please share knowledge by posting and updating HowTos in the Wiki, and post discussions and questions here in the forum.