At this point in time, virus Scanners for Linux are aimed at detecting and disinfecting data served to Windows hosts by a Linux file/mailserver. This can be useful to help stop the spread of viruses among local, non-Unix machines. Due to the lack of viruses for Linux, there are presently no scanners (that I'm aware of)to detect viruses within the Linux OS, or its applications. Trojans present a greater threat to the Linux OS itself than do viruses, and can be detected by regularly verifying the integrity of your binaries, or by using a rootkitdetector.
Is Linux vulnerable to viruses?
In a practical sense, no. Technically...
Due to the design of Linux, it is difficult for viruses to spread far within a system, as they are confined to infecting the user space of the user who executes them. Of course, this is a problem if infected files are launched by root, but as a security conscious individual, you wouldn't be running untrusted files as root, would you?
It is theoretically possible for a virus launched by a regular user to escalate its privileges using system exploits. However, a virus with this capability would be quite sizable, and difficult to write. As of this date, few viruses have actually been discovered for Linux, and the ones that have been discovered aren't worth losing sleep over. This will undoubtedly change with time. Viruses do exist for Linux, but are probably the least significant threat you face.