If there was a "law/legal" stack exchange site I'd post it there, but it's in its infant stages in Area 51. I'm a musician and an electrical engineering student. I've designed a modification to the pickup system of my acoustic/electric guitar that I know some people within a certain circle would be pretty interested in. I've read a lot of complaints about the stock pickup system on electronics forums, and it doesn't seem that anyone has figured out a solution, so I thought I'd make a detailed video on the work I've done. To be honest there's not too much to it, but if I post some partial schematics of the part of the original preamp I've managed to reverse engineer, can that get me into legal trouble at all? I don't plan on marketing this modification at all; I just want to share what I've done.
The best answer is always "talk to a lawyer." That being said, you could check out any kind of license agreement you may have implicitly agreed to when you purchased the item. Since you are not marketing this modification, the worst that could happen is that you could get a takedown request/notice for the video or a cease and desist letter. Odds are, the company either will not care or will see this as free advertising.
In general there are two main problems with reverse engineering: patents and copyrights. For US, it is often said that only the actual component layout is covered by copyright and you'll have problems if you directly copy it.
The patents on the other hand protect the idea itself and the patent descriptions try to be as broad and vague as possible in order to cover a much room as possible. If you reverse engineer a patented idea, the idea is still covered by patent and the company can still chase you in case of reverse engineering.
AndrejaKo is right about the patents. To prevent you to come up with some variation to the invention patents are never very specific, so that they cover any possible variant.
Also, most of the time it's not just the product which is patented, but patents are registered for every possible detail. I've seen relatively simple products listing over 150 patents. It's almost impossible that you can use anything from that design.
You can reverse engineer as much as you want to study the design, but you can't use any of it, especially not for production. It's unlikely that lawyers will hunt you down if you reproduce a design for your own use, just a single copy, albeit because they won't be aware of it.
The layout copyright is hardly relevant if you can't use the schematic anyway.
The concept of patents is disclosure of invention and make minor improvements to prior art. So you are not violating their design, but showing creative improvements . The value of a patents only comes to bear when others profit by similar results and get caught or the patents are worth selling.
This company would not likely waste their time or money taking you to court.
But for the entertainment field, it might detract from revenue to copy ideas and make free so people like Google/ YouTube censor copyright protected music etc. on personal uploads.
Reverse engineering is understanding of how things work.
I think it is not illegal when to reverse engineering anything. It's like hear the chords of a song and write it down to a paper.
Unless you use the design to produce and sell copies, if you do it will be copyright infringement and or patent violation.
But this is just my opinion, I'm not a laywer.
In fact if something is patented and have a patent number, it makes it a lot easier to reverse engineer. Because in order to have a patent granted, the owner have to provide all the information about the related patent, through a public domain.
A patent does not make anything harder to be copied, it makes it easier, it just give the owner legal rights to sue anyone that is using your patent.
A example can be found here: http://youtu.be/Fj7e3WGUKO8?t=35m37s