I know leeane is building one: Threadler.
I have built in the past a simple forum but it’s nowhere near ready to be used in a production site. Also, I’m in the process of building a custom forum for my sites but I don’t know if I will release the code (not that I’m against open source but simply because I won’t have time to manage a public project).
You could also look at the expression engine forum althought there is a licence fee (very affordable) and I don’t know how easy it’s to integrate in a custom app built on codeigniter.