We've released to the public the version of Zello, which runs on Windows Mobile devices. Below are download links:
Mobile version doesn't provide functions for account registration or contact list management yet so you have to had an account with contacts already to use it (you can create an account and add contacts using desktop version of Zello). Updated version doesn't require registration via desktop and could be used completely standalone. It should work fine on devices with 400 MHz+ processor and on Windows Mobile 2003, Windows Mobile 5.0 and Windows Mobile 6.0.
Please let us know, how it works for you in the comments!