This article points out three things:
1. If you download free software off the Internet there may be malware embedded in it. Free is not always free. Who's auditing free software anyway?
2. This is the first reported Mac botnet. Macs are vulnerable as much as PCs, they just aren't typically targeted as much. Most likely if the Mac user base grows significantly this will change. Also if Mac is not as attacked and Mac is not as on top of security like Windows and encouraging computer owners to update, Mac owners may be more vulnerable. Update your software!
3. Web sites that allow comments after their articles and don't pay attention or moderate them are degrading the quality of their sites.