That's exactly right mate. In a house alarm circuit, the switch is installed on the door frame, and the magnet on the door. The magnet is usually housed in an unobtrusive white plastic shell. The switches are designed to 'break' if a door is opened, i.e. if the magnet is moved away from the switch. This entirely suited my application! The magnet is strong enough to activate the switch if it's within about 1cm of it, so it doesnt need to be mounted very accurately to work. Rather fortunately the switch is also the exact same size as my 'chin' area so it fits perfectly. These switches are incredibly cheap too.
Here's a pic of the switch as viewed from the outside. You can also see my cheek covers made from textured rubber, and the boom mic for part 2 of my helmet electrics :cool