If the Odyssey client is the same for PDA's as it is for PC's, it's an add-on for Wireless NICs that let's you use WPA. Unfortuantely, I don't know if you have to use it in one case or another, as it really depends on the device and what it offers.
I know that I had to use it on my laptop with a Linksys WCP 54G card to use WPA on my network at home.
It's not necessary for WPA, as I use WPA with the Dell WLAN client. I believe Odyssey is more for VPN networks and is usually not necessary in the home environment.
I haven't had experience with Odyssey on a PPC yet, so I wasn't sure. I know that in order to use WPA on Windows 2000 or Windows 98, I had to install it, so thought that it might be the same in this circumstance.