As for the fire hazard, I can't really imagine anything happening, I use some THICK wire that could easily handle the 12Volt 18Amp output of my PSU (on the 12volt rail). I'll make sure I be careful though.
I'm a bit confused about the 60Hz. If I am taking a reading from my PSU on the 12VDC rail using the appropriate black wire, why is there AC? Should that not be as close to perfect DC as possible? On the crap core2duo psu I didn't see any waves or patterns at all, nor do I see any on online tests of junk PSU's.
The wires in your wall carry 120VAC 60Hz and emit electromagnetic waves at 60Hz and its harmonics. These waves then travel through the air and are picked up by the wires and traces in electronics, which act like antennas. The 60Hz fundamental frequency is visible as a relatively slow oscillation, and most reviewers adjust the timescale so that it isn't visible. The harmonics are always visible unless you use a very well shielded probe, and they just act as background noise, typically 5-20mV, again depending on shielding. That's part of why the 0.1uf cap is used and the bandwidth is limited to 20Mhz on the scope, so the results aren't skewed by the very high frequency EMI caused by that and also by radio/telecom/TV broadcasts, which will always be present to some extent. We don't care about those factors, since that's picked up in the wires/cables, not int he PSU. We only care about the ripple generated by the PSU itself.
If you're skeptical about these effects, take a guitar amplifier, guitar, and a cheap instrument cable that isn't shielded, and go into a big city. Turn the gain up on the amp and then start moving the guitar around and loop the cable in various areas. Eventually you'll pick up a radio station. It's happened to me; playing guitar, then bam, crappy rap music coming out of my amp. And if you do the same thing near a high voltage power line, you'll get loud static and hum.