I have a rather unique position in this regard as I work as an Adjunct at my local university in addition to working for a private employer exclusively for Government clients. As such, I have a foot in all three camps, so to speak. My experience with people employing across these fields is a little different to the conventional academic, and as such will differ from the other answers here.
Your PhD qualification (in the private sector) may actually be a hindrance to you getting employment there. A controversial guide to professional hiring in the private sector is called Smart and Get Things Done by Joel Spolsky. In his essay, he suggests that PhD graduates are certainly smart, but not always able to 'get things done'. As I said, this is a controversial book but this one view from it seems to be an element of the book that is seldom a dissenting point.
I've worked with a lot of people with PhDs and I have to say that I'm one of the few who is a dissenter. Those that enter the private sector know what's involved and operate accordingly. Those same people however also don't make a song and dance about their qualification. Generally speaking, the practice is that you only use 'Dr' in your name when you're presenting at an industry conference or standing in front of a client that is looking to you for advisory services; anywhere that requires an 'eminence agenda' to be satisfied.
The other thing to remember here is that the private sector is NOT staffed according to qualification. You will find many out there in positions of responsibility that have worked their way up from nothing within the company and find people with qualifications a threat.
I would actually recommend some research; go on linkedin, find out what you can about the person or people responsible for the decision on whether or not to hire you. What do they list as their qualifications? If there's no PhDs in that group, then you need to make a decision about whether to list yours.
Ultimately, the question mentioned in Thomas' answer holds true, even if my perspective on it is different to his. What will your employer do if they find out later? Well if you've proven yourself at the job and demonstrated that you're an asset to them, nothing. They might be a little pleasantly surprised, not because you have a PhD but because you have a PhD AND get things done.
Now for the question of ethics; I've 'dumbed down' a CV before and I don't find the practice unethical at all. Your CV isn't a report card; it's more like a business card. It's meant to represent you in the best possible light to your potential employer. Many (including myself) have a practice of producing a customised CV for every job application, tailored to what I know about the company, the job description, and the people who will ultimately be interviewing me.
I'm not advocating lying on your CV by the way; that would be highly unethical. In the modern world though, CV's are not meant to be a complete history of your work experience. They're meant to showcase that part of your work experience and qualifications that are relevant to the role. Your Masters degree is sufficient to show you're smart, but what in your work experience shows that you can get things done? It doesn't need to be a complete list but it does need to showcase what (recent) experience you have that's relevant to the role, and it should present that in a manner that the interviewers can relate to.
There's no hard and fast rule here, and you'll note that I'm going to great lengths to ensure that you don't infer a YES or NO answer from what I've written. Ultimately, every employer, every PERSON will be different in how they relate to a PhD qualification. Some employers advertising for Masters or higher will value the PhD if you include it. BUT, I've also seen people removed from candidacy because of a PhD and a perception that you won't relate to their workplace.
So to summarise, my view is that not including your PhD is NOT unethical, but the final decision has to rest with you after you've done your research on the role, the employer and the decision makers within that business.