Interesting points all.. Although if someone just wants to see what they would look like or likes them but does not want to get involved with the drilling and wiring, they make wireless ones too apparently -
http://pages.ebay.com/link/?nav=item.view&alt=web&id=131311665851
This is the same concept most security alarms use for door and window sensors. The only downside is that you have to mount it where it's visible and that might look a little tacky.
TGIF finally!!