has any one put a light under their hood.  If so what kind of switch did you use?

An alarm "pin" switch - it grounds the return from the light when the hood is lifted.

I saw the style that uses a mercury (tilt) switch available in the aftermarket some time ago. You may want to search for one of those type lamp assemblies. Open the hood, and they come on automatically. They were used as OE lights on some cars. EASY hook up too!  Mercury related items may be getting hard to find due to the hazard of mercury though.

A simple dash area mounted toggle switch would work too. Just tap into any "key on" power or off your running lights so that you don't inadvertently leave the light on (under the hood).  

An alarm "pin" switch - it grounds the return from the light when the hood is lifted.

this idea is the way to go....when the hood is lifted the light goes on when the hood is shut the switch is compressed by your hood and shuts the light off..... Having a mercury related device under your hood of moving parts and temp changes is not suggested

I own a 1987 Volvo that comes with an OE mercury ball micro switch. It also comes with a manual switch on its side, to switch to either auto or manual. It works fine and I don't see this as something of a hazard or anything.

I don't see a mercury switch as being a problem, but for a home brew job, pin switches are going to be way easier to pick up.

I agree with fordem, one advantage of using the ones from the old volvo's is the water resistant qualities inherent on the lens/housing.