I came across the strangest problem that took me a while to figure out. I'm using a C2D Mini with Snow Leopard and a Logitech Cordless MediaBoard Pro.
Being a wireless keyboard, after a certain amount of inactivity it goes to "sleep." If I use the command key to "wake it up" and it reconnects to the computer with the command key held down, it sees the command key as pressed even if I let off, meaning every click I make turns into a command-click, every key I press is suddenly a command-press, etc.
This never happened in Leopard, I've been using this setup for well over a year now and this only showed up after I upgraded to Snow Leopard.
I've started waking up the keyboard with the escape key or trackpad when I remember, but sometimes after some inactivity I try and command-tab to switch apps and it messes it up. I have to turn the keyboard off, wait 15 to 30 seconds, then reconnect. It's really inconvenient. Any ideas on ways to fix this?