IMHO the issue arises because it has already become non-trivial to buy a new system off the shelf with Xp. E.g. all the Dell web-shop offerings in UK are with Vista. Now, doubtless a few phone calls and being prepared to do a deal (and perhaps wait) will get Xp for a few months but the demand for hardware power makes Vista very attractive to system vendors. I remember that Win 98 disappeared very quickly. By announced policy, I believe Xp will not be available to most people from Microsoft Jan 2008 (and not available to anyone in 2 years time)
The investment in training, parts libraries etc. makes these very short timescales. At the university I think we have 500+ seats, a user population of at least 750 mechanical engineering students and a lot of video and printed teaching material. We are on WF2. So it is much more than a $100,000 question 😞
All application vendors ought to make their currently supported products run on Vista as a matter of urgency (i.e. like at launch) - Betas have been about for a long while after all. People who do this are the "good guys" for me when evaluating suppliers.