6 years ago, Microsoft agreed to invest $150 million in Apple, as well as continue to develop Microsoft Office for the Mac platform. That agreement ended last year, has not been renewed, and doesn't seem likely to be renewed at this point.
On top of this, Apple has been developing and recently announced its own web browser, Safari, and its own presentation software, Keynote, which, compete directly with Microsoft's Internet Explorer and Powerpoint.
A recent article on OSopinion discusses these events, what they might mean for Apple, and speculates where it may be heading. An interesting read.