You down with DDC (Yeah you know me)

Display Data Channel is the way both analogue (i.e. SVGA) and digital (DVI) computer monitors communicate back to their graphics cards what their abilities are - what resolutons they can run at and what refresh rates. Additionally the monitor will send it's name so that the driver can load colour correction lookups etc.
Now, Scene Double (link above) are my favorite KVM extenders - I've banged on about them before (see here) and they now have a very cute trick where the extender can back-propogate the DDC data from the monitor - very useful particularly when extending Mac monitors (where you can't force generic monitors like you can under Windows). Now, Ray Gordon - the lead engineer at Scene Double helped me through a problem today - I had a MacPro being extended over cat5 to a Dell 2407 monitor. The Graphics card (nVidia FX4500 - so no slouch!) refused to drive the monitor at anything above 1920x1080 even though the Dell's native res is 1920x1200. It transpires that OS-X has a problem that relates to the underlying X11 (Apple version of Xfree86) graphics engine which caches the DDC data from the first display on the card. If it has had a sniff of a different monitor (or the same monitor fed over DVI) it won't allow you to set a different resolution/refresh rate and have it stick through a reboot. The answer is on single monitor systems to hang the extender off the 2nd o/p - then it's all good!