Security firm F-Secure has unearthed a troubling trojan for Macs that hides itself as a PDF, only waiting until the file is opened up and displaying some Chinese characters before it dives into your Mac's hard drive and sets up a backdoor control. Currently, according to F-Secure, the backdoor doesn't actually do anything harmful, but obviously that could change in the future, either if the original hackers take advantage of the trojan, or if someone else does.
F-Secure says that the trojan currently doesn't have an icon associated with it, so in the current spotted form, it should be pretty easy to identify as a virus (especially if it shows up in just a random email). But if the trojan is embedded in a file with an extension and an icon that matches a familiar document type (like a PDF, or any other kind of file you'd open in everyday use), it's possible that the backdoor could get installed.
In other words, you've got to do what you should always do on any computer: beware of any file downloaded from an untrusted source on the Internet, or any email attachments coming from a sender you don't know or recognize.