Gone is the day when a phone was a simple analog device incapable of being compromised in any way, excluding a wire tap. These days, PDAs are running full-fledged operating systems and are potentially just as susceptible as PCs to security exploits and other issues - especially with their explosive growth in popularity. Apple, for instance, is coming under fire for failing to keep the iPhone up to date, which is powered by a embedded version of Mac OS 10.5.
According to one researcher, numerous flaws exist on the iPhone, many of which are the result of the iPhone's software packages not being kept up to date in the same fashion that patches for the desktop iteration of Mac OS X are. Not that it would be easy for Apple - software updates on a phone aren't as simple as pushing out an update and waiting for someone to reboot. It probably won't be long before monthly "patch cycles" for phones are common.