Is it really necessary to unplug devices from the wall before connecting HDMI?

I was under the impression that HDMI was built to be hot-swap ready, like a USB device. I was told recently that unplugging HDMI while devices were powered on was risky and could actually cause hardware damage.

Sure enough, my PS3 manual states that you must turn off and completely unplug both the TV and PS3 before connecting/disconnecting the HDMI cable. Same instructions in my digital camera's manual.

Is this really necessary to completely unplug devices from the mains, or are they just covering their butts?

Turning the power off is easy enough, but it would be a huge hassle to unplug my wall mounted TV just to connect my camera for a slideshow.

Wikipedia states that it is indeed hot swappable. No need to worry when plugging it in. Sony is just "covering their butts".

Hot pluggable Yes

HDMI also has a pin for detecting when it's plugged in:

Pin 19 Hot Plug Detect (All versions) and HEC Data+ (Optional, HDMI 1.4+ with Ethernet)

Well... HDMI cables come with a hotplug detection pin... Article

... which is recessed and will make the make the last connection upon being plugged in, and break the first connection upon being unplugged. But that's cable design.

In practice, it depends completely on the quality of the equipment that is being connected. You could see no damage, you could see ports cease to work until devices are power cycled, or you could actually damage devices due to a connection surge... even with just the 5v dc on the line.

So, yes, Sony is covering their buttocks, but there appears to be just cause.

I think you'd be especially vulnerable to troubles with hdmi hot swapping if, like me, most or all of your outlets are not grounded properly. I have my receiver/tv setup on one side of the room, and my PC plugged into an outlet on the other side. Depending on the "weather" sometimes the voltages on the shield/ground of the HDMI cable can get elevated relative to the chassis of the PC. Unless you can be sure the grounds connect first (which does happen %99 of the time) I can see popping a signal input or output if the elevated signal "seeks" ground through the chip. They should all be peppered with clamping diodes on-chip and hopefully outboard as well... but even so, the diodes can't handle any real power or they'll smoke, as well.

The latest electrical code here in the US calls for a "single point" to ground all power and communication services. I bought one, huge chunk of aluminum that looks like it's designed to be attached the grounding rod, and some other holes that look like yeah, maybe you could envision stripping the plastic covering off of some coax and pass it through.

Unfortunately all my mains wiring is tight in metal conduit. Normally, all this conduit would be grounded simply by virtue of the fact that it's all screwed together... but the idiots who replaced a lot of the early 1900's wiring with the conduit in the 70's didn't go "the last mile" with the grounding. They placed a (grounded) metal box in the basement below each outlet, but left the last 12" or so of knob-and-tube wiring in place... stupid stupid stupid as it's trivial to use the old wire to fish through fresh romex.

Which of course begs the question as to why I don't work my way around and make the final links myself. Well, grounding the system through the conduit is frowned upon...in fact may be against code at this point. As I understand it, for a long time it was frowned upon to "creatively" ground any outlet, say by just shoving a green wire into each box and clamping these all together in a grounding "tree" or ring that gets tied back to the common grounding point. But I think there also may be new exceptions: if you nylon-tie grounding wires along the paths of the conduit so that's its really obvious what's going on at any point, I think it's recognized now that that's a bit better than a houseful of ungrounded outlets. The official solution, for a long time, has been to replace all the ungrounded outlets with GFI outlets. But even those allow up to 5 ma to pass before they pop. Definitely enough to blow your speakers if you've cranked your amp, and though 25 mW (5 volts, at 5ma) doesn't sound like much I'm certain it's enough to overheat and burn up the on-die diodes in the chips. Perhaps high quality switches have outboard diodes which can handle a good 1/4 watt or more, and may be why some people have difficulty with cheap switches and some people don't.

There are other things I don't like about the wiring so ultimately I'd like to replace all the current conduit with new a size or two larger and do it up just right.

For now it's really temping just to drag some 12 gauge ground wire around the room and bond all the chassis grounds of the equipment together. And maybe tie it back to the common ground point. I think that would get rid of a lot of the low-level humming and the occasional disconcerting "tingle" I get when I'm fooling around with connections from my A/V gear