Would it be unethical to take a full time job and quit 8 months later if I get accepted for a PhD program? I am applying this winter for graduate school and I will finish undergrad this winter as well. Would it be unethical to take a full-time position with the intention to quit soon until I know whether I got accepted for graduate school or not? 

I am considering this because if I didn't get accepted, I wouldn't have a gap until I start working, and the company would be a good brand which would certainly not harm my PhD application (but this shouldn't be a discussion on what prepares me good for graduate school). My plan is going back to industry after a PhD, so considering my reputation in the field or possible research jobs lateron in the same company, how could I reason my leave for getting a PhD? Or would you better decline the offer and wait for the admissions result?
It wouldn't be unethical for your boss to fire you 8 months after you take the job, if he wanted to. Labor is an exchange you and your employer enter into voluntarily. So, you shouldn't feel bad quitting after 8 months if you want to. It's not an ethical problem at all. It is more plausible to think there might be a personal problem if this is a small industry that you would plan to re-enter after finishing the degree. Don't tell your boss up front that you might quit in 8 months, that isn't any of his business. If you do accept an offer to do a phd somewhere, then you should tell him promptly, just as a matter of courtesy. This is to allow the company adequate time to find a replacement.