Dran,
You're absolutely right -- Linux is not immune to an "email attachment" virus, especially those that require user interaction to propagate. I also think you're right in that Linux is a "small fish" compared to Windows as a target writing a virus.
The Windows environment is a mono-culture with respect to mail readers and address book structures. There are a few mail readers available for windows, but Outlook has by and far won the battle of the mail reader. This makes the Outlook address book a very easy one to target (and definitely the "big fish" to go after).
The mono-culture example is a very important one in many industries. Take agriculture for instance. In agriculture, farmers are encouraged to not just plant one type of corn in thousands of acres of crops. They are encouraged to mix different variations of corn in the field. This way one "virus" doesn't take out the entire crop supply.
If you look at Linux in this light, you'll understand that your "small fish" idea is even more relevant than you originally thought. Under Linux there are literally hundreds of popular mail readers. This makes it much more difficult to create a virus that will spread at such a rapid and successful pace.
If you think about it in this manner, you'll notice that it actually breaks the one Linux "small fish" up into a school of even smaller fish, each fish requiring a slightly different type of bait to be caught. Now, I'm not saying you can't make a big net to catch them all. I am saying that the net is just much harder to build - and much easier to wiggle free from.
Now add on the fact that most Linux and Windows mail readers support hundreds of advanced "convenience" options, many of which make the spread of a virus easier. But the difference is, the Linux mail readers usually force the user to consciously make those tradeoffs. Under Windows the tradeoffs are often made by default - without asking the user first.
One final point about the virus risk. Operating Systems like Unix (and to some extent Windows NT) provide a good defense against the damage a virus can do to your system. If the user who executes the virus cannot write to the system binaries, data and configuration files - typically the virus won't be able to either.