Related link: http://www.cw360.com/article&rd=&i=&ard=113969&fv=1
For users and companies who are relying on VBA macros or other customizations built into Word, Excel, etc., they might have a significant reason to stick with Microsoft. Plus, there are the external applications that use the MS Office Object Models, creating a dependancy on MS Office for anyone that wants to use that application. In either case, the ability
to use existing bits from Microsoft may save enough in development costs to justify maintaining the status quo. CW360.com offers more thought on this issue.
07/12/2002 - As noted in the comments below, there is API and macro functionality with OpenOffice. For more information take a look at http://api.openoffice.org/. On the ToDo list is a wrapper for Microsoft-Office API emulation.