OK, so many people...
...have overlooked the fact that their "phones" are in fact nothing but portable computers. Many times more powerful than the "high-performance" XTs and ATs I started to work on in the mid-1980s. Still, in my experience the major reason for the success of malware of any kind is the sheer idiocy of the users who will cheerfully and without even stopping for a moment download stuff and start it up, without any verification of its origin.
As Mark65 has pointed out, a secured marketplace does have its advantages for those who unquestioningly double-tap on files/attachments named "pornviewer.exe" or similar. The problem is not the device, it's the gullibility of the user. Which (unfortunately) cannot be stamped out. Spam would be a thing of the past if people would only stop reacting to it -- spammers only turn a profit if they generate sales. The same goes for dialer trojans (or, for that matter, all trojans). One may point out that the standard settings for the Windows Explorer -- to hide "known" filename extensions -- further this behaviour, and I agree with that. And, for a quote, "few things are as uncommon as common sense" (though I don't remember by whom that quote is; I have read it in literature dating back to the 1950s).
Particular to the dialing trojan "problem, in a sense, the telecommunication companies are partially at fault; a very few have already introduced cost-controlling measures such as an easy-to-enable option to allow only a certain amount of charges per month when dialling a selectable range of numbers, after which all other attempts will be cut off. And the customers of those companies are usually not being told that the option even exists. I have enabled the option for my contract; my bills have remained the same, though.
As with email spam, the widespread use of such measures would kill off dialing trojans, IMHO.
Batten down the hatches. My best guess is that most people impacted by malware are not the type who are reading el Reg, but as a professional IT consultant, I will give you the completely free advice to review your security settings, from firewall through AV all the way to the way you handle downloads, mail attachments, and other people's data sticks. With some self-discipline and proper OS settings, I have so far found that most AV software is basically superfluous (really!).