Good information, and not a bad idea with the GFCI suggestion. Although shy of water contacting the AC side I can't foresee a scenario where this would offer protection from something that would actually happen.
Even though the case is not metal, there is LOTS of metal within each computer component. A capacitor is formed every time you have 2 or more pieces of metal separated by an insulator. The capacitor will "charge" (and "leak") over time. When this capacitor gets shorted (for any reason), then there will most likely be an imbalance between the hot and neutral currents...hence popping the GFI breaker.
Also, I thought maybe earth ground and the supply's DC ground were separate, however think of this; When you measure with a DMM between +12V and the steal chassis, you'll get a reading of 12VDC.
A DC reading is RMS. If you want to see the AC component, then you need an oscilloscope to see the AC noise on the DC line. If you do this, be careful as the "clip" on the scope probe is connected to earth ground...and you can toast a circuit by connecting earth ground to something it's not supposed to be connected to.
Also, if you were to connect a fan to the 12V, and attach the ground to a steel part of the chassis, the fan will run. So somewhere the two do meet each other. Or is there another explanation for why my observations are the way they are?
Yes, the fan will run...my first electronics professor had a saying: "Voltage is BS...current is what does the work". (He was Russian, so this sounds VERY cool when you say it with a Russian accent). Voltage is just a potential (to do work)...current flow is actual work being done. You have to follow the current path:
(1) Out of +12 V on power supply
(2) Into Fan + connection
(3) Through Fan motor
(4) Out Fan - connection
(5) Into steel chassis
(6) Who knows where next
(7) Somehow to power supply case
(8) Somehow to power supply DC ground
I prefer not to have the unknown of 6-8 myself.
Basically I ask this because I'm wondering if when lacking the AC ground (3rd pin, earth) are you at a greater risk for damaging the system with ESD when you physically touch the IO, or plug something into the MB? Or is the PSU's floating ground capable of discharging that ESD from your fingertip and keep the system safe?
Bingo! It's about safety...for YOU first, then your equipment.
Just take something as simple as a network cable. Side 1 is connected into a switch (which is connected to earth ground). You want to connect Side 2 into the Ethernet jack on your PC...but the case is floating...when you plug it in, you will cause a short and will have a current pulse through the network cable as the charges equalize. How much of a current pulse? Shrug...depends on the voltage potential between the two and the resistance.
Electricity is dangerous...it can hurt you...bad.