Apple is really good at hype (see iPhone). Unfortunately, they tend to contradict themselves and over-simplify complicated issues when it suits their goals. Actually, lots of companies do this, but Apple sticks out to me because of their tongue-in-cheek methods.
You’ve probably heard Apple rag on how insecure Windows is, but then you may have also heard how awesome it is that you can run Windows apps on a Mac.
Guess what? To run Windows apps on a Mac, you have to run Windows in a virtual machine. Here’s a news flash for you: your computer isn't so secure anymore. Watch out for those 114,000 viruses infecting your computer.
All sarcasm aside, you shouldn’t have any problems with your Windows installation on a Mac if you take steps such as keeping Windows up-to-date and running a virus scanner. However, it is much easier to become vulnerable running Windows in a virtual machine. It’s also a good possibility that if you run a Mac, you may not know the steps needed to secure Windows.
When using Windows as your main OS, you probably run it almost every day. This allows Windows to fetch its updates, and for your virus scanner to do its thing. When your run an OS as a virtual machine, you may or may not boot it on a regular basis. I know this happens on one of my computers at home — I run Linux as the main OS, and run Windows via VMWare every few weeks.
Since you may not boot your Windows install on a regular basis, you might not get those updates in time. You might be more vulnerable than if you ran Windows only.
Moral of the story: You need to be even more concerned about Windows security if you run it on top of OSX. Perhaps someday you won’t need to run Windows to use Windows apps, but from what I have heard, darwine still has a way to go.
In case your were wondering, I currently don’t own a Mac (just two linux boxes and a Windows XP laptop). However, I must say that a Mac mini running Leopard and the new iMovie does look very tempting for editing and cataloging my home movies.