Here in Washington, this will be treated like the flu vaccine. We have case law here stipulating that an employer can require a flu vaccine as a condition of employment. It is rare to see mandatory flu vaccines outside of healthcare. Most healthcare employers that do mandate flu vaccines allow for the employee to opt out, although that usually comes with restrictions: wearing a mask full-time on the job or transfer to a job with no patient contact (if such a job is available, and there are no guarantees you will like that job or get paid the same).
Locally, it will be interesting to see if Boeing mandates it for production staff on the factory floor. It is not easy to run wiring in the fuselage while staying six feet away from each other.
PS: at our COVID briefing this afternoon, we expect the first doses of the Pfizer vaccine on the 15th, and the Moderna vaccine on the 22nd. We have a committee deciding which employees will get first dibs. Probably those who are most at risk to being exposed to aerosols from patients: the emergency room, anesthesia, respiratory therapy, and critical care nursing. The committee will expand from there. As an executive administrator with relatively little patient contact, I will be at the tail end of the line, and it all depends on how many doses are allocated to our system.