There's a great article from The New York Times today about mobile device exploits that, combined with increased processing power in mobile phones, could create mobile botnets.
Botnets, of course, are loose affiliations of compromised computers (networked in that they were all compromised by the same hacking effort and are controlled remotely by the same group or individual). What's the danger of botnets, besides that they are computers over which owners don't have full control? The main problems is that these computers can be used to initiate Denial of Service attacks, but the common application is that the botnet controller (some call these "bot herders", and I quite like that term) uses these computers to send spam. Now, it seems, botnets can jump to smart phones like the iPhone or a Blackberry.
Fortunately, the consensus is that most of these phones are worth hacking now, but that as the phones get ever more powerful and add more and more functions of our core lives (and as we replace those bulky desktops with a device in our pockets and purses), then these phones will increasingly be targeted for hacking and bot nets. I'm summarizing the article, so just do yourself a favor and take a minute to read the original NYTimes.com article on mobile botnets yourself.