Would I use one? Yes, but I would also let the person know. Having said that I would only use it if the person was unknown to me, and I didnt have anyone I know available to babysit and I had no other choice by to leave my children.

In my opinion it is different then FDC or daycare in that the person would not be around other adults, by making the person aware of a nanny cam it kind of acts as a moral anchor another adult would normally provide (i.e keeping temper under control, no hitting/biting the child). Having said that, a nanny cam would only pick up once an act has been done, bit to late if any serious damage was done.