Proper "notice period" for resigning a tenure track position?
Hiring an adjunct or borrowing someone from another department for one course is about the least bad thing that can happen in this world. Giving the kind of notice (9 months?) that would have allowed them to do a full candidate search is impossible for folks who move laterally out of a department. This is pretty common and departments simply have to manage it. Everyone knows that offers are being locked in during the April-May time frame, and sometimes you lose someone key. It would not be unprofessional if you chose to avoid delaying your career by staying for an extra semester in order to make your current department's process painless. 

Nobody wants to see you go, but you're working within the system to the best of your abilities. You're not screwing them, and it's not unprofessional to wait to give notice until the new offer is accepted.