wikis yes. forums.. not really unless you abuse the bug tracker. Github+Google Groups seems to be the complete package these days. BitBucket also provides website/wiki/issue tracker/source control features.
Yeah, it's probably a good solution for new projects. The problem with existing projects is when you already have a quarter million posts in your forum and don't want to lose them. Although maybe a mix of providers - keeping forums on SF and moving the rest of the project might be a solution.
Forum is one area Github is in a great position to enter right now. Wrangling bug tracker to act as a forum is a bit of a hack so a proper place for developers to interact and discuss projects would be most welcome.
I have a product I'll be launching soon in exactly this space: mailing lists/forums/chat with github integration.
Email me if you're interested in alpha/beta testing (see my profile).
GitHub certainly has wikis, I often use project wikis to document various bits and bobs.