||Here's how my ethic judgement work in this particular case.
I personally believe that ethicality on quiting while in at-will employment, depends on how much the company is willing to accomodate you. The ethic is 2 ways street.
If the company expect me to never quit while in the middle of chasing the deadline that define the company future, I would expect them to never fire me while I need money the most (for example: to cure my dying daughter). If the company expect me to never quit suddenly after so much they have invest on me, I would expect them to never fire me suddenly while I'm undergoing an important life change that assume ongoing employment with them. In short, the minimal/worst limit of my behavior is: I will do unto them what they do unto me.
The problem is, we will probably never know how a company will treat us in different circumstances. Maybe we can see similar incidents with other employees in the past, but we will probably be treated differently for whatever reason, either be treated better or worse. At the end, you can choose to be nice and expect they return the gesture, but always prepare for the worst.