well i did think that myself, but a modem is most likely easier to emulate in the software than a motherboard and CPU is, don't you think?
also, it depends on the incentive. there are hundreds of different modems out there that are supposed to only work with windows, and many of them have linux drivers now, how many different windows only computers will there be? one. or maybe two or three different models. Plus, if some organisation (like mandrake or red hat) decides that their distro will be the first to work on this new platform, then the sky is the limit.
While we're at it, let's not forget that linux has been ported to TWELVE or so different CPU architectures in its time. I have yet to see Windows running on an Alpha or a Sparc.