Not really for pocket PC yet, it's like macs: not because they're secure, but because most viruses are written for money now, and the rest are written to do widespread damage.
If you're building a l33t virus, would you put the effort into the 97% of the market using PCs, the 3% using Macs, or the 0.001% using pocket PCs?
(Also, before virii and trojans can be written, people need to find exploits, and again the effort involved with finding pocket PC exploits gives so little vandalism "fun" in return.)