Of course, security is relative. Last month, the folks at Coverity released some data for defect scans on 31 open-source projects. The average defect rate for 1,000 lines of source code was 0.42. Not bad. A programmer would, on average, crank out 2,200 lines of code for each flub. But if that rate were constant against, say, the 30 million lines of Red Hat Linux 7.1, you'd have 12,600 lines with problems. If it held steady against the 213 million lines of source in Debian 3.1, you'd find 89,460 potential defects.
This isn't to say that Debian is less secure as a server operating system than Red Hat. Or vice versa. But it does point to the kind of information you can use to lower the risk your information faces. That is, you can use tools to quantify your risk and then decide when, where and whether to use a technology.
Common Sense
You can also use common-sense strategies to protect your company's information. Would your data be inherently more secure if more end users had Macintoshes? Despite news in February that the first (benign) virus for the Mac was discovered, the answer would have to be yes. That's because viruses and worms written for one system wouldn't be propagated by the other. In other words, a mix of operating systems is a good defensive strategy.
Do all end users really need fat clients -- Windows or Macs? Would some be able to get their work done more securely on thin clients? Of course. A mix of thin clients, Macs and Windows, as well as different server systems, is an ideal defense against many of today's vulnerabilities. A side benefit is challenging the skills of hackers who will try to penetrate your defenses with primarily Windows-specific knowledge.