We do this, at Five Across, and our business is practically run on the collaboration and file sharing that is provided by our Bubbler Instant Messenger application.
But yesterday we made a more important transition: now we're serving our own dog food too.
We moved our main bubbler.com site from Apache to our own Five Across web server. So the whole site bubbler.net is served by our web server, not just the blogs themselves. It is how we structure the product for our OEM customers, so we use it too.
This is a "transition" because we built out the bubbler.com web site before our own software was ready, so we had two sites (bubbler.com, which was powered by Verio, and bubbler.net, which was our blog hosting server). Now the whole thing is powered by our Five Across Web Platform, and it's performing beautifully. It handles server-side includes. It handles multiple IP addresses per server. It is a total replacement for Apache, yet it does way more. Blogs, file serving, podcasting, instant messaging -- all built directly into the web server itself--compiled C code, blazingly fast. It's THOUSANDS of times faster and more scalable than our competition, scripted things like Moveable Type. Perhaps 10's of thousands. We're planning to do some performance benchmarking soon.
This is getting to be really fun.