In a post on Microsoft’s Windows 7 blog mactopia it is announced that they are moving from Carbon to Cocoa for the user interface of Office for Mac 2011 for a better performance. The main problem has been the lack of compatibility between the Windows and the Mac version of Microsoft Office. The problem was already being discussed when Office for Mac 2011 was introduced (Feb. 11, 2010), and Microsoft stated that:
“Nowadays, compatibility means more than just file formats,” Microsoft’s Kurt Schmucker told Macworld. “It’s also workflow, collaboration, and user interface.”
Office for Mac 2011 is assumed to be released some time this year.
Source: http://www.officeformac.com/blog/Office-for-Mac-Drinking-Cocoa-at-WWDC