Do you guys have a USB hub connected to your Macs?
It could have no relation at all but here's what I went through just a couple of months ago. I am still on Tiger and this happened running Tiger btw so there might be no similarity at all with your problem. I bought the new Alu keyboard 'cuz the one that came with my iMac G5 wasn't pleasant to use and some keys were getting harder and harder to type with.
I plugged in this magnificent new keyboard and it worked fantastic for the first day. Then the next day it started to be erratic: keystrokes would not register, or the letters would appear with a one minute delay, keys suddenly would go on typing by themselves like if I continuously pressed on them (to make it stop, I would have to unplug the keyboard).
I tried reinstalling the driver, all sorts of troubleshooting things, to no avail. Out of frustration, I took the keyboard back got a brand new one in exchange, I was certain it was a bad keyboard I got the first time.
I plugged in the new one and blam!, it went berserk on me as well. I was so disappointed: I was going to take this one back as well and ask for a refund. I then started thinking and unplugged one by one the USB peripherals and when I got to a certain USB 2.0 hub, all the strangeness stopped. There was an incompatibility between the new Apple Alu keyboard and the USB 2 hub!
Anyway I might be completely off the wall here, but when you installed Leopard, did you take the precaution of unplugging all the peripherals except the necessary ones to install the new OS? That is something to consider with all software updates or upgrades, including not using the Mac while the Software Update process is underway.
Hope this helps.