Obviously the giant monster sitting on under or near your desk is going out of style. Laptops nowadays are becoming powerful enough for the average user, and even gamers in many cases, and while the desktop form factor will likely be around for ages to come....its easy to see that the markets are concerned with portability. So, what does this have to do with phones? Well, many things. First of all, modern cellphones are personal computers in the purest sense of the word. They perform tasks that would normally require a lot more work on our part, such as calling people from wherever we are, tethering to your computer for internet, banking and so many other things I can't even fathom. Add to this the fact that there are a staggering number of smartphones out there (14.7 million were sold in 2010 q2 alone) and you have a question that I pray to god someone brought up in the development meetings for blackberry, iphone, and android "Are they secure?".
For the most part, yes, "apps" run in sandboxed environments in most cases and on operating systems such as android, the apps fully disclose what they want permission to access and a confirmation box has to be ticked by the user, even if its a newer version of a previous app. Apps, due to their sandboxed nature, cannot access the storage space or memory currently in use by another app (I.E. a game your playing cant look at your bank statement). However, while this is technically secure, there is a serious problem. People. When you are storing your bank account information and social security number and all of that on a desktop or laptop with PROPER antivirus and very strict user access controls/application firewall, you don't have to worry so much about it (although the latter I would suggest keeping off digital media period). Now think about what would happen if that computer was a few inches in size and could fit in your pocket? Think about someone swiping it off of a counter or table? Your lack of a password, hard drive encryption, and clear thinking have led to your device being compromised despite your investment in security software.
What is my point? Keep sensitive data off of portable devices wherever possible. Your social security number, and credit card information have no business being on your smarthpone. If you are in a situation where you absolutely have to have them, make sure your device is password protected and you have purchased software that allows for remote wiping of the phone. Many security suites offer both cellphone antivirus (a whole other subject) and features such as phone tracking, remote wiping, and etc. Just keep this rule of thumb in mind; whenever you install that app on your phone that can unlock and start your car, and whenever you put sensitive data on your phone, just take a fraction of a second to think about the possible consequences. Cell phones get stolen in staggeringly large numbers, its not a risk worth taking.
So, don't go investing in $50 worth of security software unless you ABSOLUTELY need to keep sensitive data on your cellphone in the first place, this also applies to tablets as well....seeing as the operating systems on them aren't much more than glorified smartphones themselves.