I would like change wording here: Apache is the most popular internet server around and thus a very interesting target to find exploits for. It is not more prone to worms as any other software.

But from the bugs Apache had (which were fixed very timely by the Apache developers, by the way) I have seen only Linux Intel exploits. It is also possible to use buffer overflows and similar ilk on Mac, but due to the different CPU architecture are not portable easily. I did find a documunt named PPC_OSX_Shellcode_Assembly.pdf which appears to be a tutorial on how to do this on Mac. So may be the worms will also spread to Mac in the future.