on ritzcamera.com they have it listed as an accessory for the D40 but on amazon.com they have it listed saying its for the Nikon D50 & D70. Does anyone know for sure if its compatible for the Nikon D40?
just to add amazon.com has it for $13.99. thx for the info 
It sure does. I own one for my d40 and its a great add on. Very handy for night and studio shots. Probably one of the best additions to your camera for $20. My guess, though I don't know for sure is the reason that the D40 isn't always listed with the remote is because the remote came out before the camera. This remote will definatly work (just remember to go into your settings and turn it to the remote setting). Good Luck and have fun with it.
http://www.bhphotovideo.com/c/product/207373-USA/Nikon_4730_ML_L3_Remote_Control_Transmitter.html
Your camera's instruction manual has a list of all the compatible accessories for your model camera.
Nikon's link says: "Compatible with N65, N75, D70, D70s, and D50 Cameras" http://www.nikonusa.com/template.php?productNr=4730&cat=1&grp=56
Yet, Samy's shows "ML-L3 Wireless Remote Control for D40, D50, D70, N65, N75, Coolpix 8400, 8800, Pronea S, Nuvis S & Lite Touch Zoom Cameras" http://www.samys.com/product_detail.php?item=6943
I will guess with the differences of data, taking your D40 into your local camera store and test it will be the only sure way to know. I use a Nikon MC-30, which works just fine for my application
References :
digiPro