I need to extend a USB device from a corner of my room to the above the fireplace. Since I'm already going to be installing some HDMI and audio cables through the wall, I'd like to have my USB device run through the same wall plate; I'm not having any luck determining if it's safe to do so, whereas it's quite easy to find in-wall rated HDMI or speaker cables. Would this extender be safe to run through the wall?
If you are going in-wall over 16 feet, you need an active extender as many have pointed out. One alternative to this is to use a Cat5 extender: this allows you to run Cat5e/6 in-wall (riser or plenum cable is rated for in-wall use), then convert to USB at both ends as needed. One I found at Monoprice is rated for 150ft.
If you are using USB 2.0 and your run is more than 16 feet, then I would suggest not to run it. USB 2.0 standards specifies that the maximum length to be not more than for 5 meters (16.4 feet). USB 3.0 does not have a length rating but the wire gauge (26 AWG) limits it to 3 meters, or 9.8 feet. Source is from Wikipedia.
EDIT gregmac is bang on with running the cat5. 0A0D is right about the safety end. It's like speaker wire in wall. You can not run just any speaker wire in the wall, it has to be in-wall rated. As for the extenders in the wall, I really don't think any wiring, even low voltage with coupling or connectors that are not accessible should be run in the wall.
The reason I like gregmacs answer is that cat5, with the right connectors can be used for a multitude of things.
I wouldn't run USB cable in a wall if it is not rated for in-wall installation. You could release toxic smoke or cause a fire if not done correctly.
One option you may not have considered is a Wireless USB hub. It's less intrusive and works well depending on what you are planning to do.
Generally speaking, most low-voltage wires are OK for behind-the-wall wiring.
Is it safe to do run USB in the wall? Yes. Does it follow the standard? Possibly. The standard sets the limit at 5 meters, so if it is less than that then you should be good. USB is notoriously testy about current flow. While it may work for your current setup, when you try the new USB 3.0 at high speed you may get some nasty data loss. Not to mention the EMI from the other cables. If you insist on running it through the wall use a high quality cable that is shielded. If you need more technical info the full spec is here Good luck!