OS News has an interesting piece on why, with all the Sassers and Netsky's and other Windows security issues more companies don't just go with Apple:

This isn't the first time that those widely-publicized Windows security issues have bitten this company. When you think of both man-hours trying to fix the problem, and the combined loss of productivity in a company this size, the cost must be amazing. So the question must be asked: how can this company -- indeed, any large corporation -- rationally choose to support a Windows infrastructure?

The answer is complicated, and has as much to do with inertia, ignorance and comfort level as it does with dollars and cents.

Among the reasons cited: Enterprise IT hates surprises, Apple doesn't have a dedicated enterprise IT sales force, enterprise IT wants solutions not 'stuff.'