about once a year for the last few years some a**holes think its funny to steal whatever thy can out of all the cars in the neighborhood. Well it happened again on Sat. they didn't get much, but just for the principal of it, it pisses  me off to no end.

I want to put in a 150 Halogen Flood light on a motion detector, (also will put in a photosensor to make it only come on at night). This will light up my driveway at night since it's dark around my house.

My question is; Is there any restrictions or codes about mounting this light to a chimney? (we have 12" square block chimney that runs up the side of the house, and is a perfect location)

If Not I have a plan B but it probably would not look as nice.

Different municipalities have different codes.
A call to the local building department would be a good idea.Face to face would be better.
I personally wouldn't attach to a chimny.
It just seems to be asking for trouble.
If you just "bootleg" it in make sure that it is at least GFI protected.
You can do this by replacing an outlet with a GFI outlet and take power from it.

Dont really want to put up a "temporary" light, I intend on installing 1/2" EMT (at least for the outside stuff)  

As far as GFCI, all my outside outlets are on 2 GFCI breakers, because the GFCI recepticles dont seem to hold up well in an outdoor environment. Even with the outdoor covers on them they still somehow manage to get moisture in them.

That shouldn't be a code issue. As far as emt outside, I would ease away from that unless you are using compression fittings. Even so, they are not watertight and you'll get a lot of leaks at the couplings and connectors. It will start to rust in about 2 years even though it is galvanized. I suggest pvc, but you are going to have to strap it quite often or it will droop down once it heats up in the summer. No conduit is 100% watertight due to condensation anyway. Don't run any romex or UF cable exposed either. Even though UF is rated for sunlight, it will eventually break down. If your doing it yourself, you shouldn't have to worry about the city's code unless you just want to pull a permit. One last thing, most motion detectors have a built in photocell so you shouldn't have to provide a seperate one. Might want to install an overide switch so you can manually operate it or turn it off.