Originally Posted by carnageX
This is true; it's actually the better way to ground yourself to do this than just touching the case metal.
That's pretty much correct, you aren't grounded unless the system is plugged in, think about it, where is the line to the ground? When you touch the case you are bonding to it, equalizing the charge between the two bodies so minimizing the possibility of a static arc...all the components are bonded until plugged in.
Personally i don't plug in, I'd rather deal with static than the psu being plugged in. The one system I've seen fried was off but plugged in, my father in law's, i was talking him though unplugging and removing the HDD but he unplugged the monitor rather than the power supply...snap, crackle, pop.
"Josh....there is smoke coming off the power supply and something smells burnt. I think something went wrong...."
Fried everything except the HDD ironically.