I have to say that .AI is not .AI!
There's a zillion versions of .AI. If I would put .AI on the web, I would either do it as version 8 or lower of .AI (about the last real AI version) or as editable PDF (currect style AI) with PDF extension.
AI version 8 can be read by many vector packages like Inkscape, Freehand, CorelDRAW. Editable PDF as well, but it might loose layer-information.
If you have a modern AI that does not open, try renaming it as PDF, as that is actually what it is.