An article I've read says the iPhone's default security system is effective, but the same cannot be said for jailbroken iPhones wheres hackers can get around the iOS security system. A lot of the major anti-virus companies have developed mobile anti-virus protection software. Not sure about Android and Blackberry smartphones however, my apologies there.
It depends how you use your phone really that determines wether or not you're at risk of getting viruses.
You're about as likely (if not less) to get a virus on your iPhone or Android device then a Mac computer. Yes there are anti-viruses available. Are they really worth it? In my opinion, no.
I have an iPod Touch, an Xperia Play and a Blackberry Bold 9700 and don't have an anti-virus for any of these devices even if my iPod Touch is jailbroken and that I download apk files on my Play. I am, however, taking risks in doing this, but the risks aren't all that big either. Depends how stressed you are for your device, but I think that if you're worried enough to get an anti-virus, that you might as well doing everything legit.
At least that's my personnal point of vue
You do realise that that article is incredibly biast right? All the information is coming from a company that makes anti-viruses. Of course they're going to say that the threat is big.
I don't deny that Android is more at risk because it's opened source, but last time I checked, Linux was also open source, yet Linux and also Mac OS, which runs on a base of unix are the two platforms with the least viruses. Hackers will more likely attack the more mainstream device, not the necessarily the easiest.