Where did BI get their numbers? Some broad assumptions, honestly. Discussions with developers who consistently have apps in the top ten suggested that top paid iPad apps will sell about 7,500 copies on a Saturday or Sunday, and about 2,500 on a weekday. At $10/each, that works out to approximately $825,000 per week. Admittedly, it's "a very rough estimate." The iWork apps have been among the top 10 paid apps since the iPad's release in the US.
An estimate, yes, but it does illustrate an important point: users want "serious" work apps for the iPad. Sure, the initial user base was largely fans who already enjoy the iWork apps, but that fact that those apps are still among the top ten suggest that customers other than the fanboys are buying.
Not bad for three productivity apps on a platform that "won't mean business."