I was browsing over at PocketPC Thoughts when I saw this freeware personal firewall for PocketPC devices. With viruses being developed (and released) for the PPC platorm along with the proliferation of wireless-enabled PPC devices, you can only expect to see more of these apps come into fruition:

Features:
Monitors inbound and outbound network-level communication
Provides packet-level filtering
Detailed alerting and logging functions
User can quickly select security zones of varying strength
Option to ignore (drop packets from) a particular IP address (select from logs)
Connection overview lists all currently open ports and the state they are in (e.g. a "netstat" for Pocket PC) with connected IP address.
Built in protection filters (ACL) ("zones")
Ability to define custom filters
Trusted computer option based on IP address
User-adjustable DoS detection and protection (SYN scan detection)



Source : AnandTech