Then it turned out I had the wrong DVI/VGA adapters, so I bought some from Fry's, and they were the wrong ones, so I brought them back and got the right ones, and then I was missing some cables I needed, so I bought those. Then I hooked it all up. I've had the switch since before Flipside and it's taken like a week to get all the materials together so I could try using it.
So I tried to hook it up and I realized that the DVI-I video cable that I bought wouldn't plug into my monitor. At that moment I finally understood DVI. There are two standards, analog and digital, and they can coexist on the same cable. If they coexist, it's DVI-I (for integrated). If it's digital only, it's DVI-D. Well, my monitor had 2 ports, VGA and DVI-D. The DVI-D port is missing some sockets for the pins which the analog signal is carried on, so you physically cannot plug a DVI-I cable into that port; it won't fit. OK. But if I were to use an adapter to convert the DVI-I cable into VGA and plug it into the VGA port, this would basically drop the digital signal and the MacBook Pro video would be lost (because it is on DVI-D).
I determined that to solve this, I needed a new monitor which supports DVI-I. There are such things as video converter boxes, but they cost $100+, and I figured it would be simpler to take this opportunity to upgrade. (Since then I have learned that there is at least one other solution, to use a video splitter and appropriate adapters to plug into *both* ports at the same time. But I didn't consider this.) At Fry's I found a lovely Samsung LED 23" with 1080p resolution (SyncMaster XL2370) - I had never heard of LED monitor technology before but I was impressed - the contrast and clarity were striking. So I bought it. There's $350 I hadn't planned to spend, but what a nice monitor.
Got it home and hooked it up. Hmm, unlike my old monitor, the new one no longer fits in its box now that I've attached the stand. Mild annoyance - I like to keep the box for moving and such. Hooked things up; cables and adapters everywhere. OK, MacBook Pro worked great. And... the VGA windows computer didn't work.
WTF?? The computer was detecting the resolution of the monitor, and let me move my mouse over to it, the screen was just blank. I hooked up the computer directly to the monitor, and it worked fine (yum, nice image quality) but I noticed that it had to switch modes from digital to analog. Was that significant? No idea. Hooked it back up to the KVM and same result: blank screen. Tried forcing the monitor to switch modes and still no dice.
I called IOGear tech support. They were slow on the uptake. The guy finally told me I couldn't use VGA cables plus a VGA-to-DVI adapter to hook up my VGA computers to the KVM, even though that's exactly what the instructions said to do. They said I needed to use DVI-I cables and the opposite adapter, or else it wouldn't work. I was extremely skeptical, but what could I do except spend another $60 on cables, try it, and call back when it didn't work? Grr.
After doing some more research I had a couple of other ideas. Either the port might be bad (one of the NewEgg reviews described ports going bad) so I could try using a different port, or I could try this mysterious video card imprinting thing using a sekrit key combination (numlock-minus + D).
If I try all of this and nothing works, I will return the KVM and go back to square one. In that case I will feel a little guilty for buying a new monitor for no useful purpose, but at least it's a really nice monitor.
Resolution: I went to Fry's and purchased 3 DVI-I dual link cables, figuring if those didn't work, nothing would. (I still have no idea why you would need dual link vs. single link, because I haven't found a single straight answer on the web, but I was using single link up to that point and that certainly wasn't working.) Shocker: that was all it took, everything suddenly worked. Hooray!
Now my only issue is that when I switch from the Mac to the PC, there is a delay while the monitor searches for the signal (for some reason it has to check HDMI before it checks analog, and each source it checks takes like 5 seconds). Don't think there is any solution for that.