Found a solution courtesey of Grant Adams on the Apple discussion forum. He said "Simply open up the exported ppt on your Mac. Then "save as" and change the name slightly. Then take it into your windows environment and open it in Office 2007". Works a treat.
After creating my presentation in iWork08 Keynote I save it first in its native format, and then in PPT. When I try to open the PPT in Office 2007 on a Windows machine, all I get is an empty Window. If I open it in Office 2003, no problems. If I then save it in Office XP/2003 format, Office 2007...