Uhh, that would be Microsoft Office...without that, the Mac would be toast...even for home use.
Interestingly enough, the most installed app on Windoz computers is iTunes. Go figure?
What I do suspect may happen is that iWorks on the iPad might actually break the world's addiction to the tasty MS Office suite... just sayin' that could be how the end of the world would come to Redmond WA.
Try writing a 100,000 words novel and crunching those huge product and financial Excel sheets on the iPad and its touch "keyboard". When you can honestly tell me that you've enjoyed this experience, I know that something must be wrong with you.
The iPad is a pure viewing and consuming device. It was not designed for creation or (text) input.
I also doubt that a device that is coupled to a restricted and censored AppStore is going to be more than an initial niche success.
Google's Android platform might not be as shiny as the iPad's iPhone OS yet, but it is Open Source and has a completely unrestricted market place. Furthermore, everyone can build his own hardware around it. That mix of a consolidated software platform and unlimited hardware variations is what made the PC so successful -- and at the same time the total vendor lock-in made the Mac the niche platform that it still is today.
History is going to repeat itself here, only that this time the competition is not called Microsoft Windows, but Google Android and Google Chromium OS.
And eventually, the open platform is always going to win. And that's something that Steve Jobs still doesn't get. In a few years from now, he'll do the same whining again how everybody in the industry stole "his" ideas. And as we all know, Apple had stolen most of its ah-so innovative ideas from others (e.g. GUI, Mouse, touch screens, now even the whole iPad concept), they were just great at catching all the hype.