I would tell your boss soonest that you are moving (is it for a spousal job transfer or a family issue or what? Not my business but if it is for your spouse's job transfer they may indeed be very accommodating).
I think you should tell him you are moving and present it as a fait accompli and say you value the work you do for this company and would like to set up so you can do it by telecommuting. Tell him the gift of time will allow lots of opportunity to restructure a few tasks to make it work as well or better than things are working now and you are willing to, the last three months, work a day or two a week from home, just to start testing the waters and work out any bugs telecommuting might unearth while you are wrapping up anything at the office the other weekdays that need your personal attention.
Also, this gift of time will allow for the purchase of equipment you may not now have available, like teleconferencing equipment for you to virtually attend staff meetings, etc. and for you to consult with the network engineers and start building the network if needed.
I think if you are indeed the senior IT person that they will want to keep you any way they can and you should not worry about job security.
However, if they choose to pass, your comment that you "can't leave the company in the hands of the only other developer" sounds a bit smug and controlling. Maybe she doesn't have the capacity for your job because you don't delegate any management tasks to her and she has not had the chance to try it, or maybe you are right but either way the decision to move her into your position or hire outside the company if they decide not to have you as a telecommuter is the company's decision alone.
I do see a problem, if you are the senior IT person and in charge of day-to-day management of the department they may in fact remove some of your management duties simply because you are not on site and you cannot always do management by phone and email.
Also, if the office environment and relating to the people was a plus side of the job you might want to consider applying for similar positions in your new city. Telecommuting can be lonely and isolating and you may find yourself out of the loop after a while.