I recently watched this fascinating talk by Jonathan Zittrain, author of The Future of the Internet — And How to Stop It. He covers everything from the Apple II to the iPhone and trusted computing. His basic premise is that malware is driving the resurgence of locked-down, single-purpose devices.
I disagree with that conclusion. I think malware will always infect the most valuable platform. If the iPhone was as widely-deployed as Windows PCs, you can bet people would be targeting it with keyloggers, closed platform or not. In fact, the motivation of people to find ways around the vendor’s protection on their own phone leads to a great malware channel (trojaned jailbreak apps, anyone?)
However, I like his analysis of what makes some open systems resilient (his example: Wikipedia defacers) and some susceptible to being gamed (Digg users selling votes). He claims it’s a matter of how much members consider themselves a part of the system versus outside it. I agree that designing in aspects of accountability and aggregated reputation help, whereas excessive perceived anonymity can lead to antisocial behavior.
rdist 
The mass of malware will go to the dominant platform. But malware authors will also write for obscure systems at least once, just because. The latter efforts are usually harmless, though.