I founded and run an Open Source PHP content managment platform which we've been developing for the last 7 years or so. We made MODx originally because creating pure HTML-CSS driven sites with zero creative limitations or making assumptions about how your content should be structured just wasn't available. If you know HTML/CSS (and JS if you want it), you can create a template for MODx ... at any rate, we're about to publicly launch a company we've formed and it's time to update our forums (currently in SMF with 27,700+ registered members and 310K+ posts ... not huge, but not small either).
At any rate, if we transition to XF, we'll create a bridge and make it freely available to anyone that wants it. That way you could have a blog, and an ecommerce store and a marketing website and much more along side a great forum. We need to see the code first and to understand the templating/theming engine, moderation/board administration and authentication APIs before we decide. I've been following this project and the lead devs certainly know what they're doing, so I'm definitely optimistic! We're trying to decide between XF, sticking with SMF even though it's impossible to tell when 2.0 will ever come out of RC (*grumbles*) or IPB.
As an aside, things to be very careful about with bridges is the licensing with other Open Source projects. I think that pretty much anything that interacts with some of the projects mentioned in this are required to be licensed under the GPL. We take a different approach at MODx, because frankly we don't view interaction with a public API as something that should "infect" another project with a viral GPL license. I guess we're a bit of an anomoly in the traditional Open Source world because we really are very pro-business when it comes to licensing (see http://www.thrash.me/tech-and-modx/open-source-vs-open-core.html).
