There are really few known virus, at this time, designed to atack mobile O.S., some of which are proof of concept 
only (which means that they do not represent a real danger for the Pda users but only an experiment to check which
ways and technologies can be used to attack a mobile system.

THE FIRST VIRUSES DISCOVERED
Its name is Duts (or Dust) but the possibilities that it can hit your Pda are low, almost as the probability that you open
the door to a thief by your own willing. Read HERE   
You can find details about this virus HERE  
There are others viruses that are not a proof of concept as the above unfortunately.
Brador A, is a Backdoor that can allow to the virus author to load further dangerous code.
You can find details about this virus HERE
Liberty Trojan is a Palm O.S. virus. Distributed as a crack for a known game that can fully reset you pda.
You can find details about virus HERE   
Phage is another Palm virus that can adamage all pda's files so that you need to hard reset device
You can find details about the virus HERE
You can read HERE a resume of actual Viruses' state. 
Cabir is virus created to infect Simbyan system thru bluetooth connection
You can find detail about virus HERE
CommWarrior.a e/o Mabir-A are viruses that can use MMS to infect others Simbyan pda phone
Infojack Trojan per WM5 / WM6 the first real virus coming from Cina. Some informations HERE

The above can suggest that Pocket PC Antivirus Becomes a Necessity as you can read in this CeWindows
article (the test includes also links to learn more about mentioned PPC viruses). 
Notwithstanding their article i consider the actual status for Pdas users need to be of Attention only as shown
by the Yellow Light signal shown on top. (The light will turn RED when it will be necessary to use antivirus)
In the meantime HTC in order to prevent the risk got an agreament with C.A. to add Antivirus to their PDA.See HERE
Obviously this is my opinion only and somebody may wish to use now an antivirus program to prevent problems,
so you can find here a list of the available Antivirus for Pda:

Producer :   AIR-SCANNER
Download:   Antivirus
                    Firewall
Manual    :   Antivirus Manual  
                    Firewall Manual
Note        :   This is a programs' suite to protect your PPC and it is FREE.
               :    Last version

Producer :   ANTIVIR
Dowload  :   From the producer's page directly update and documentation included-free 

Producer :   AVAST
Download:   PPC-Arm
                    HPC-Arm
                    PALM
Note        :   This programs appear quite old (there are also SH3,Mips,SH4 version) and not valid for WM O.S.

Producer :   BIT-DEFENDER  
Download:   From the producer page (available for WinCE e Palm O.S.)  
Note        :   The program is FREE         

Producer   :  FB-4 INC (VirusGuard)
Download  :  from this Page  
Note          :  Shareware. The licence is valid an Year only  

Producer    :  F-SECURE
Download   :  Trial apparently available for Smartphone only HERE
Manual       :  Antivirus Manual
Note           :  http://www.f-secure.com/estore/
Nuovo         :  http://mobile.f-secure.com/fsc/retail/
                     Nuovo per PPC. Disponibile sia per WM2003SE P.E che per WM 5.0 e per pocketPhone e smartphone
                     Versione prova 30 gg. Può essere direttamente scaricato connettendosi col ppc/smartphone  

Producer    :   KASPERSKY
Download   :  Trial version HERE
Note           :   Documentation can be find from the  Download page. Commercial 
                  :   Version 2.0 released. No trial available for this version. Details HERE  

Producer     :   TREND MICRO 
Download    :   Trial version HERE  
Information :   See this Page
Note            :   Commercial version. The free-trial expire June 2005   
News           :   NEW Beta Version HERE 

Producer    :   SYMANTEC 
Download   :   Trial Version   
Review       :   Program's Review HERE  
Note           :   Downloading the trial version you subscribe their new letter.   

Actual conclusion (February 05) :
The risk to get a virus infection for your PDA is quite LOW.
Anyway your device could be another channel to infect your Desk computer.
If you use your PPC to check the mail and thence transfer the same (attachments included) on your desk
computer the transferred files can include some x86 virus without any direct effect on your pda.
First of all always update your Desk Antivirus whatever you are using.

(Subject to update)

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