Well, according to Winn Schwartau's article of Feb 27 in Network World, Macs may have a MUCH lower TCO. He even lets you download his spreadsheet and examine the equations or even tweak it for your own needs. For his example, he claims that his costs would be halved with a Mac environment instead of WinTel.
While I am a big Mac fan, I don't agree with everything Winn says. In the first paragraph, he says "Imagine PCs that are close to immune to the endless train wrecks caused by viruses and worms." Recent history (last month) taught us that Macs are, in fact, not at all immune. The simple fact is: Macs currently enjoy the perks of a small market share. Translation: Compared to WinTel systems, far fewer people are busily devising ways to break into Macs, and this is because they want to have a big impact. They want to affect the most people. Or ... maybe they just hate Bill Gates.
Anyway, to finish my point ... when Apple's market share increases (and I think it will), they will certainly fall victim to more attacks against their OS. It's normal, and I'm sure that they already know this. In the mean time, I will enjoy these joyous days of relative secure bliss using my macs.