I heard on BBC's Digital Planet podcast, that, in 2008, there were 1 million malware incidents, but only 6 on cell smart phones! Unfortunately, the podcast is no longer available and the source wasn't cited.
This was given as an example of how secure smart phones have become.
Well, the two most common smart phone architectures (Symbian and Blackberry) use a centralized architecture, which is easier to secure. Importantly, telephony is isolated from the data side.
The world is moving towards peer networking and VoIP, so security (I suspect) is not a permanent attribute of smart phones (unfortunately).