Most PSU's have built in discharge circuitry to help prevent electrical shock from occuring. If te PSU as never been plugged in and turned on, its capacitors (the things that retain the electricity) will never have charged. If the unit has been turned off and unplugged for awhile (> 5 min), then the unit should be safe.
As mentioned previously, DO NOT wear rubber gloves. Just don't go sticking your fingers where they don't belong (lol). Seriously though, just make sure you have discharged any static build-up before handling things and wear your grounding strap if you have one, you will be fine.