The only reason I could come up with this is - Some mac lover just got too high in the Autodesk food chain....
It's pretty simple. The 8% and growing US Mac ownership is hard to ignore, and the 27% which own or plan to own iPad, well AD would be insane to not have a presence on that platform.
Back of the napkin here, but
100 developers at $200 hr (salary, benefits, etc.)
100 misc others involved in the port project @ $200 hr (salary, benefits, etc).
2 years development time @ 2080 hrs a year.
$2000 per seat AD pockets
200 * 200 * 4160 = $166,400,000 to develop and bring to market.
So about 83,000 seats need to be sold to break even. Can that number of seats be obtained in the first year? How about by the third year?
Anyway, it's about money and market trends. Autodesk is late getting back into the Mac market, but in the long run it won't hurt them at all. Not unless they lay an egg with this first release. Stay with Windows, switch to Mac, doesn't matter either way, Autodesk makes money.