I can't find any information about whether the client and server are proprietary but Virtual Bridges claims:
1. The client/server protocol is an open source extension of VNC.
"The VERDE Clients Protocol utilizes the Virtual Bridges VDI remote client protocol, an open source implementation combining the standard RFB (VNC) protocol with remote device access functions. This mechanism allows clients to remotely display server sessions even over low-bandwidth connections, as well as perform local device functions such as printing and audio playback... Given the open source architecture of the VERDE Client Protocol, the mechanism is completely extensible and can easily be amended to support new devices and platforms."
2. You can access your virtual desktop using an ordinary open-source VNC client or X11, albeit perhaps without all the bells and whistles of the "rich client". According to a quote in The Register, the client can
be "any device, wired or wireless, that talks X11, VNC, RDP, or runs
x86 Linux or Windows and can run our rich client application. This
covers most every type of popular client out there, including PCs,
laptops, workstations, thin clients, as well as today's subnotebook,
wireless, and mobile PCs."