Its interesting to see what everyone has said.

Yes, Windows has the biggest market share (not a big surprise to most people), and therefore more viruses , worms, and trojans are written for it.

However, we (i work for an AV company) are seeing more and more malware being written to attack specific organisations, or for a very specific task.
It looks like the days of the mass mailers, and extremely quick spreading worms are over.

The really attractive targets now are those that can make the author money.
Ransomware for one, and trojans to cause a DoS on a companies servers so that a competitor can profit from it.

I would think that the next big target is going to be something like Office 2007 with its new file formats (basically zip files with an xml file index). Office 2007 will use the same file format as OpenOffice, so this makes for a possible way in (if an exploit is found) on multiple OSs.

Im slightly off topic now i know, but if you want a target, then you cant get much better than infecting all the machines on a network regardless of OS, rather than just the Windows ones.