I had contemplated this idea, for some time. Everyone has a giant chalk board it seems. But then it also seems that everyone has much bigger houses. So this was where the dilemma lied. We had no free wall space to have a clearing and paint one up, not in any of the rooms. The only free walls were in the hallway and as its so narrow, i hated the idea of constantly brushing past chalky walls. So... last resort....the back of the front door! Its a great place to have it. We also have a draught brush at the bottom of the door which handily keeps the hallway floor clear! (You can get dust free chalks but i found they scratched the paintwork as they are really tough.) Anyway, an old flannel and pot of chalks later and my daughter can practice her numbers all the time, without my help to get the pens and paper out. The top half is handy for me to write reminders to myself too. But of course i still have plenty of lists flying around the house as well!
One of the most important things to do if you paint up a wooden door is to sand it back properly first as the paint will chip off all too easily otherwise. As the chalk board paint will not stick to the woodwork paint.
Another job off my list.
(Once you've painted up you board, make sure you season it fully with chalk first. This avoids any permanent marking from your first drawings.)