As others have said, the walled gardens are *EXTREMELY* safe. iOS App Store and Google Play are both *VERY* safe.
Jailbroken iPhones are targets, but most people concerned with open platforms are on Android - and sadly Google has gotten people used to "going off-reservation" for some apps. (Is Kindle Market available to install direct from Google Play yet? Or do you still need to root and side-load?)
Symbian is effectively dead (the former leader of malware,) and Palm is all but buried at this point. Not sure about CrackBerry's ecosystem. Microsoft's is basically as safe as Apple's.
That leaves Android as the only reasonable target for malware. Sort of like how in the '80s, Macintosh was the primary target for viruses, as it was the most likely to be networked - then as Windows got internet-connected, it became the prime target.