I first investigated the idea in this posting to the Movable Type support forum where I postulated that you could make Invision PowerBoard the comment management system for an MT-powered site using pings to map MT entries to Invision threads.
I think this is a great idea, but the other day I got to wondering why you couldn’t just run the entire site off of Invision and skip MT entirely. For the index pages, just take the first post off of each thread. Subsequent posts to that thread would be comments. Invision allows you to set granular permissions on forums, so registered users could start new threads while guests can only reply, thus duplicating the functionality of posting and commenting in MT.
Comments in MT are kind of an afterthought, really. They have limited functionality because that’s not really the point of a blog. Blogs are oriented to one-way content (read from publisher to reader) rather than two-way (publisher and reader have a discussion). In contrast, look at sites like Slashdot and Plastic — commenting and discussion of entries there is kind of the entire point.
MT has “categories,” Invision has “forums,” so that functionality would map right over. Invision offers the added perk of a hierarchy — parent-child forums instead of the flat landscape that MT has (this is one of the most requested features for the next release of MT — I understand that MT Pro will have this).
You’d lose the pretty URLs of MT, but I think you could hack this back in with a mod_rewrite or AliasMatch directive.
Invision allows the option of user accounts — which some people like to track comments more tightly — while still allowing anonymous posting. Since user accounts are built in from the start, there’s great support for user administration. Unlike MT, you could allow certain authors to post only to certain forums, or only allow comments in certain forums to registered users.
Additionally, Invision is written in PHP and MySQL, which is generally more friendly to hacking than the Perl backend that MT is built on. With this, you could extend the platform further and get away from some of the limitations that you tend to run into with MT (user-defined fields, anyone?).
I have nothing against MT, of course, but using discussion forum software is an option worth pursuing if you’re starting a new blog and are looking at software. Invision is rendered via skins which you can download and plug in. I don’t think it’ll be long before someone offers a MT-ish skin to make Invision a blogging platform right out of the box.
In fact, I may do it myself.