So, I'll note a few things:
The forum has a nice, eat your own dog food feel.
You can subscribe to the RSS feed.
The forum is tricky to authenticate to.
To post topics, you have to be approved by a moderator. Does that really scale?
The mailing list is a known technology that just works.
I can authenticate to the mailing list with my email identity.
The scale tips toward the mailing list in my view. Any thoughts?

I don't know... I tend to prefer forums for support. It provides a user experience that is more ammenable to people searching for answers, and I am willing to bet is more Search Engine and SEO friendly.
That being said, I think it is important to have both email and forums. But email is a medium that I think is more appropriate for certain audiences, like developers and hackers.
I think forums are fine - but I'm not sure whether this incarnation is ready for production (although I can see that it will be nice once the edges are smoothed off !)
There are other minor nitpicks (reply permalinks, the profile page doesn't link back to the forums, some of the older topics aren't updating properly - I left a reply here: http://forums.movabletype.org/2007/11/welcome-to-the-forums-powered.html and the last reply is still showing as November 2007) - but I think the above have to be sorted before you get tons of people using the forums.
Also - I had quite a few problems posting this reply - was getting an error (that I didn't note down - sorry). So I signed out and back in again, and this time it's worked
I concur with everything Anu has said. This system is rough. If the RSS included replies and new topics with links to comment through. I'd load it up in Google Reader and never look back.
I need the ability to post without moderation. How does one achieve that status?
I have mixed feelings :)
Mailing lists - I always check my mail
Forums - I have to make the effort to visit them
I left a long winded reply on the ProNet on this subject, I won't go into it all here.
I will say that a good forum notification system can eliminate mneylon's concerns. If:
Then we get the best of both worlds. I get the organization, indexing and attractiveness of a forum with the immediacy and delivery of an email list.
I did not see a reference to what the forum or mailinglist was going to be used for so its a bit difficult to answer...mailinglists have their needs and forums have their. It all depend on what you want to use them for.
I totally agree with Byrne... A forum could be searched... Or be indexed by Mr Google... This is a very goog point when you need support.