That's probably why there's not a single decentralized forum yet. And without centralized server, everyone will need to keep the entire database of all posts. It won't run in a browser, so no advertising for SEO either. Search engines don't visit decentralized forums.
Theymos wrote about the possibility of a decentralized moderation system, kinda like the current Trust system. So if you trust me, you won't see posts I've deleted. But if someone creates a billion spam accounts, that's still going to be a problem.
I expect to be stuck with a centralized forum for a very long time.
Everytime I'm reminded of this, I'm always thankful to the people and organizations that are running those servers for free or at a really low charge just so we can establish a server that's outside of what the common Internet users go to. Regarding decentralizing a forum, what's the big deal of not becoming one because the centralized one seems to me a more appealing thing because that means there's more people that can easily access and participate to the discussion and most of the time, these types of decentralized forum's got something to hide.