Let me break it down for you.
URLs are data--that's it. There is nothing magical about them.
Computer programs can systematically manipulate data.
Part of IPB's conversion to FURLs would be a program to systematically manipulate the URL data.
No Apache required. No IIS required.
You guys keep talking about web servers and other stuff that has no bearing on a data conversion UNLESS the conversion is dependent on an existing mod.
With respect to exactly how this would be implemented, you appear to be missing the point. Of course IPB would be able to handle FURL's quite simply, and that would be manipulating data, and using the database as you describe. However, what is in question here is how you could create a friendly URL implementation that works across the major server software providers.
To create a URL such as mydomain.com/forums/My_Topic_Title_Here.html, you would need some sort of server software level rewriting, to allow your script to be called. Implementation of rewriting is massively different between the softwares and between the versions of each.
To create a URL such as mydomain.com/forums/index.php/My_Topic_Title_Here.html, your server software would need to provide PHP with the $_SERVER['PATH_INFO'] variable, to my knowledge, Apache is the only one that does this.
Of course, you could use URL's like mydomain.com/forums/index.php?My_Topic_Title_Here, or index.php?topic=My_Topic_Title_Here, but that's really not a lot more friendly than simply using query string URL's in the first place.
There really is no other way you could manipulate the URL used to access it via PHP alone. The server software in use does
have a bearing on how this could work. Unless of course you're suggesting that all existing posts in the database be loaded, and cached to individual HTML files based on their topic title.
Of course, you don't have to take the word of me or any of the other web developers who posted here. Infact, I'd be beyond happy if you showed me up with a working, coded example of how to do this cross-platform, regardless of server software and configuration.