Currently there is almost no malware for the Mac that is a serious threat because it is very hard to write malware for the Mac. However, when Mountain Lion (OS X 10.8) is released this summer, the challenge presented to writing Mac malware may become insurmountable.

Apple - OS X Mountain Lion - Gatekeeper keeps your Mac even safer.

Apple already sees to it that software in the Mac App Store has been checked to make sure that it doesn't contain any malware. With Mountain Lion (ML) Apple will see to it that even software that you don't download from
the Mac App Store has a measure of security. Legitimate Mac software developers
will be issued a unique encrypted certificate code.
That code will allow ML to check to see if individual apps have been
modified (i.e. infected) and it will also allow Apple to keep track of
malware and to keep malware from running on ML via a nightly update.
These steps may see to it that Mac users
never have to give a moment's thought to malware ever again.