There are at present two viruses which have been written specifically for Mac OS X, these are:

http://www.sophos.com/virusinfo/anal...simpsonsa.html
http://www.sophos.com/virusinfo/anal...plsfromra.html

Both of these viruses were pretty usless when it came to actually doing anything, but they did bring forward some interesting ideas.
Trojans are something that could be written for OS X very easily as its more down to the user having to actually run the file. So user error is more to blame with trojans, and considering there are something in the range of a little under a thousand viruses for unix/linux of which some would run effectively on OS X, and some which would partly run and probably do more damage than they were meant to in the first place because of the changes in Darwin.
Worms however will be something that will be very few and far between for OS X. I cannot think of any method apart from things like 'Apple Remote Events' and 'ssh' which would allow a program to install itself to a Mac, but this could change.
At present Mac is a secure platform because no one is writing any viruses for it, but there is no reason why this could not change.
I work for an anti-virus company and of the 98+ thousand viruses that we detect something in the area of maybe 100 have anything to do with Mac specifically. At the moment Windows is the target, and in my opinion will be for a long long time to come.