Apple's ecosystem of software and developers is too small (64%)
Perceived difficulty (52%) in integrating Macs into corporate America's Windows-centric infrastructure.
A small installed base and therefore a limited track record in business (41%)
Smith doesn't address small and midsize companies directly, but these reasons certainly apply there as well -- along with price, of course.
The post also references a story in the NY Times over the weekend, which asserts that Microsoft's Vista problems offer a temporary opening for Apple, but that a lack of emphasis on retailers and resellers means that "The Macï¿¼s share of personal computers has actually edged a bit lower since Vistaï¿¼s release in January."