First, I like to thank you for sharing you work and knowledge to the public!
I found your page today and I think I found exactly what I've been looking for. I am a web designer on Lanzarote/Canary Islands Spain and work a lot with the fantastic Open-Source Program for Real Estate websites -and listing sites in general- called Open-Realty(TM).
This listing script has a fantastic add-on system and a lot of developers and programmers develop nice little addons for it to make it even better. There are already some different addons to make two ore more database fields and values dependent on each other (like: country/province/city or for car listings: make/model search fields) but the 2 addons are not really good. The first requires core changes so it is not an addon, the second works as well with Ajax but the XML file doesn't get generated by real database values, it must be handwritten instead.
Now that I found your Ajax Linked Select boxes, I see a possibility to maybe make my (and many others!) dream come true. Do you think you could give me a hand on this?
If you are interested, I will send you more information about addons for Open-Realty. Or maybe it would be easier to just modify the existing Ajax add-on and you explain me how to populate the XML file automatically that the addon uses.