That depends...
Since cross platform threats are now becoming more prevalent; how are you to truly know your Ubuntu platform isn't actually pwned? You have no way of really knowing, unless you're a file freak who is constantly looking at your file structure.
I agree that viruses are not really the threat here actually; but malware. Today's malware don't need administrative privileges, and can do harm without taking over the operating system. They can pwn the browser though, using java/adobe vulnerabilities and do damage to anyone who uses the machine for financial purposes. Quite frankly for my clients that do not use their PC's for shopping, banking, or E-Trading, I tell them not to worry too much about viruses/malware/etc. Because they have little to lose, and as long as they don't click on fake alerts and run as a limited rights user, they will be fine. Especially if they run CCleaner before every log off and/or shutdown.
