As Linux gets more and more popular & finds a large user base, we can be sure that the creators of virus programs will turn their attention towards it sooner than later. Already there have been some outbreaks of virus aimed at Linux. Most notable among them are:
1. Linux/Slapper
2. Linux/Lion
3. Linux/Ramen

Not much was heard about the outbreaks of these viruses as is generally experienced with viruses that attach Microsoft systems is mainly because the Linux user community as compared to Windows users is very very small and the user base that uses Linux in a corporate environment with huge amount of critical production data is even smaller. Most of the users using linux use it as a laboratory environment for testing or as an hobby same as yours truly does.

Of the above viruses, Linux/Slapper has hit the Linux user base the hardest, with about 14000 computers getting affected by it according to one industry report (http://www.virusbtn.com/news/latest_news/granneman.xml). Although the number is not very high when compared to the numbers reported by Windows users affected by virus, it warns the users of the things to come in the very near future.

If Linux wants to seriously challenge Windows as the preferred desktop operating systems then the open source community and the ISVs for Linux would have to pay more attention on securing the operating system. One would be very foolish indeed to ignore the signs of things to come.