It's not a matter of it being solid because it's ignored. Mac OS X actually has a pretty good number of stupid problems. It's matter of it being unexploited because it's ignored, there's a difference.
Windows makes a convenient target for worms/viruses because it's so widely deployed and so homogenous. On top of that, Microsoft's patching history is HORRID. From ignoring problems to slow turnarounds, broken patches to unrealistically large ones, many people find it very painful to update Microsoft systems. As a result, exploits for Windows tend to have a good deal of time to grow in the wild before they're unleashed. An attack on 1 Win9x machine will probably work on 90% of all 9x machines and 75% of all XP machines. The problem is that it IS exploitable in a wide environment whereas each *nix system is not - it's exploitable in it's own little way.
*nix systems, have a tendency to be far less homogenous and so aren't typically exploited by worms or viruses anymore. *nix systems require you to dive in and get you hands dirty to pull off an exploit. For example, sendmail is a very popular, very broken application that runs on many, many *nix installations. However, rare is the occurence of a worm or virus that can exploit it because it often doesn't cross BSD/Linux/Solaris/AIX/OS X boundaries. Instead, most (if not 'almost all') of the sendmail exploits require you to actual get hooked up to the computer running the service and try to break your way in manually. This is far more difficult, so requires far more skill, and results in a loss of almost all the boring canned script-kiddies that can easily attack Windows. Also, you tend to have more security-conscious code slinger in OSS (probably due to the lack of marketroids telling them what to do) who don't do stupid things like embed VB scripting in spreadsheets. Windows has always taken the attitude that it was more important to be feature rich than safe and *nix has gone the other way. As a result, you get bloated, hole-riddled behomeths on Windows like Outlook and Word, and you get lots of little applications that can be chained for functionality on *nix. Since each little peice was written independent of all the other peices, you have fewer tie ins and less opportunity for wide-spread exploits.
Exploits on *nix exist, they just can't be exploited as easily most of the time. Windows, on the other hand, by design, is very easy to break using canned methodology because of it's insistence on convenience and 'make it familiar'.
The argument that it's exploited because it's popular just doesn't have any solid ground to stand on, really. Bear in mind that the juciest targets on the web are nearly all running something from the Unix family tree and they don't really get hit by normal exploits too often (DDoS.. yes.. but that's something that afflicts ALL systems).
I might note that the only thing that can be excused is viruses sent via e-mail. There really isn't much of anything that can be done about home users opening infections unless we can get them to stop doing it (although, heterogenous *nix systems would again make this type of exploit more difficult to pull off in as widespread a way as we see Windows attacks work). But then, that's where *nix shines again - by setting the proper executable restrictions on your filesystems, admins in a corporate *nix network can prevent these infections right at the source - the user. The same can (sort of) be done with Windows now, but it's usually such a hassle because it causes problems running other things that it's not a very good trade off.
Sorry... there's really no good reason to believe that popularity has anything to do with the widespread expoitations on Windows and not *nix....