Although I'm a fan of Amulet Hotkey for sending 2 x DVI, USB & audio i/o over ethernet networks (and even slow connections; works over a domestic internet connection) we did fit an Avocent HMX system on a recent job.
Unlike the Amulet where you can do remote admin of both the transmitters and the receivers (referred to as Zero Clients) over a web interface with Avocent you have to use a serial connection (see my post about RS232 for making a null-modem cable!) with a slightly unusual comms setup.
The clever thing is that if you want to connect to a transmitter (the gadget that sits behind the server or workstation) you have to do it via a receiver that is logged onto that transmitter over the KVM-network. I suppose this allows you to administrate a very distant workstation's sender from where you are.
So - RS232 port (male, therefor a DTE, therefor a null-modem cable required) and you can see what you might not be getting any video from the remote computer.
What I discovered is that unless the EDID (or DDC if you're doing it over SVGA) profile matches the resolution of the sending machine you get no video! What is the point of that - the only reason to pass an EDID or DDC profile is to allow the graphics card to tell the system what resolution is being expect.
Anyhow - fixed that by logging onto the machine locally.
By the way - is there a decent PuTTY-like app for OS-X. Good job I keep Windows on my MacBook Pro for situations like this.