So, do you need an anti-virus application for your Linux PC? Reread what I wrote above and make the decision that best fits your own circumstances. In other words, the need is specific to your own situation. Also remember, this isn't a statement regarding Linux viruses. It's a statement about Windows viruses you're unknowingly sharing with Windows PCs.
Additional security tips to consider
Previously, I mentioned using iptables to help keep your system secure. Considering that not everyone is probably going to want to roll up their sleeves and do this the hard way, there are some handy front-end options that can make this much less intense. The first option that comes to mind is for the Ubuntu desktop and it's called Gufw. Based on Ubuntu's own adaptation of working with iptables, Gufw allows you to run a simple GTK application that offers you the following functionality:
There are also KDE options out there as well, in addition to other enterprise solutions for handling iptables. The takeaway here is that using a firewall on Linux can be as simple or as advanced as you would like it to be. You, the end user, are in the driver's seat. With the simple act of blocking ports, you can help to prevent anything undetected. Obviously, though, a firewall by itself isn't a silver bullet.
A learning experience
Personally, I've found that keeping a close eye on the software I run, along with paying attention to how my home network is configured, allows me to run freely within my various computers throughout my home. This isn't to say that the issue seen with the anonymous OS distribution isn't questionable, but it certainly doesn't affect how I seek out new distributions to try or how I install my software. I think that, more than anything, it reaffirms that security risks sometimes just happen. It's unfortunate, but it's life.
In addition to using a commonsense approach to keeping my PCs safe from malware, I also make it a point to be aware of where the distributions I install are coming from. When I seek out a new Linux distribution to try, I remain vigilant by only downloading ISO images from trusted sources.
After all, the real threat has nothing to do with Linux at all. The single biggest threat to Linux security is sitting in front of the keyboard and monitor. Sadly, there isn't a security suite designed yet that can protect us effectively from ourselves.