According to many sources including BBC News, a worm is spreading via the Apple iChat IM application. It poses as screenshots of the new version of Mac OS and calls itself latestpics.tgz. In order to get infected, a user would have to receive the file and then actually extract the contents of this archive and then run an application inside it. The application poses as a JPEG image (by using the default JPEG icon) but is actually a PowerPC-compiled binary application (so doesn't work on the new Intel Macs presumably, or does it successfully emulate?). What it then does is does a Spotlight search to find the last four recently used applications and infects them. Then it tries to send itself to all your iChat contacts. But that's all it does, no deleting of files or anything. Well, actually according to the stuff I've read, it seems to be so badly written that it unintentionally breaks some apps that it tries to infect.
It doesn't seem that bad to me; I guess it's just hit the news because viruses and worms are so rare on the Mac platform.