To be honest, I never gave this much thought. And I'm sure lots of other users didn't either. But your handset is just as susceptible to viruses, spyware, and hacking as your PC is. So why don't we use the same level of protection on our handset that we do on our PC?
One reason is that the Internet, where PC-based attacks are generally made, is essentially unregulated. That's not the case with our handsets. All the data that's sent is done so via a network that's owned by the service provider. And in most cases, they don't want you to know if/when there's a problem.
The second reason is that the attacks aren't nearly as prevalent on the handset as they are on the PC. But some data that I saw recently suggests that those attacks could be on the rise.
One vendor that's at the forefront of this technology is McAfee (www.mcafee.com). They have a significant knowledge of this area, as they are one of the leaders in PC-based anti-virus and anti-spyware software. They have developed (and will continue to develop) a series of products that protect the handset. The products are offered at various levels, including the end user, the enterprise, and the network.