Symantec have just released Version 4 of their Mobile Antivirus solution.  This provides protection against malicious code downloaded from the Web, sent via email or a Wi-Fi connection, or beamed via Bluetooth or infrared ports.   You can also setup optional automatic scans after synchronizing with a PC or inserting a memory card.

I couldn't find details of whether it supports Windows Mobile 6 but it does support 5.0 and 2003.

You can get more details HERE