I'd love to see one of the manufacturers for these boards step up and say something about it, but not sure if they will. In any case, I bet the response is we've done testing and it's worked for us, but it isn't designed to do this.
I have mentioned this before (I think) on the topic of hot-swapping PS/2. It's totally at the user's risk, but having said that, I'm not sure how much risk there actually is with anything manufactured in the last 5 - 8 years or so. My case is far more extreme than anyone else's is likely to be. Here it is:
The system: P2 600 eMachine with just about the lowest end MB you could imagine.
I have hot swapped keyboards and encoders on this thing, no exaggeration, at least
5000 times. Once, while prototyping, I mis-wired something and watched a resistor glow bright orange. The port stopped working after that....for about 10minutes. Once it cooled down, it was right as rain.
I also do a lot of testing on different systems and can say that I have never turned one off before plugging in a keyboard or encoder. Never once saw a problem. It doesn't mean that something might not happen to you, but it has never happened with me...with PS/2.
I can relate an experience that happened about 10 years ago though. I was plugging a keyboard into my prized 486, you know, the big ugly 5-pin DIN deal, and I did indeed pop the fuse. The cause was probably related to the fact that is was one of those old, power hungry (by today's standards) clicky keyboards that most likely didn't have a resistor on the power line to help limit the draw of the keyboard when it started up, as well as a small fuse on the motherboard. It was a long time ago and with hardware that bears little semblence to what is out there today (or even yesterday).
Something else to consider: The power circuit on the KeyWiz is the same, whether for USB or PS/2 operation. Some of you have probably noticed that the only difference hardware-wise between the USB GP-Wiz and the PS/2 KeyWiz is the connector. There is a low value resistor on the power line that provides for a ramp to 5v, albeit a very short one. This may too alleviate some of that surge at plug in time.
So is there a conclusion to all of this? Not really. Just a bag of experience, coupled with some facts, a fairly loose specification for the port and the knowledge that the last thing a MB manufacturer wants to deal with is a rash of users with blown PS/2 port fuses (which works to your benefit here.) But I will try to sum it up as best as I can.
Does it work? Yes, but you have to understand the risks, just like you do every day when you hop in your car to go to work. They are quite small, but they are there. My expectation is that if you get away with it the first 10 - 15 times, you will continue to have no problems, as that has been my experience.
If the fuse pops is it the end for the motherboard? Not usually. It'll be a small pain in the butt, but you should only need to hack a keyboard extension cable and run a 5v line into it from the power supply, i.e. bypass the fuse that popped. From there I expect that you could hot-swap until the cows came home (does anyone know where the cows actually went?)
If I plug in a KeyWiz (or other PS/2 encoder) with the power on and I blow my fuse, will you buy me a new motherboard? No.

If you are using an ancient motherboard, play it safe and avoid it. Otherwise, if you are really that worried about the small possibility of this happening, you should get a couple of USB GP-Wiz's or GP-Wiz49's and a cheap hub and hot-swap your heart out.
RandyT
BTW, as an interesting aside, how many folks reading this have had a problem with hot-swapping PS/2 in the last 8 years that they can absolutely attribute to it? This may be a deserving of a poll question
