The Mobile Antivirus Researchers Association (MARA) recently published a Top 10 list of most critical Wireless and Mobile Security Vulnerabilities.

http://www.mobileav.org/

From a Windows Mobile Perspective - looking at the Smartphone and PDA section...

6. Clear text passwords - In Windows Mobile - we store passwords in a hashed encrypted form.

7. We can prevent malicious code with the Application Installation and Execution Security model.

8. Autorun can also be disabled by using the Security Configuration Service Provider Policy.