Anyone know of a reliable source of data for the % of users by Office
version?

The only good survey I could find was Martin Green's Office Version
Survey - http://www.fontstuff.com/comment/comment03.htm

Version - %Users
=============
2003 - 16%
2002 - 35%
2000 - 35%
97 - 13%

His survey is 2 years old, though. It would be great to see something
more recent.

Nick Hebb