Although its a year since this question was posted, someone else might hit on it. Hence this post.
I recently bought a Samsung UA40D5000 (Series 5 LED) and couldn't get ARC to work to my Onkyo AVR.
However, you can find "Tech Specs" for each model on samsung.com (find the model, click the Tech Specs tab) and they are, at time of writing quite specific about ARC under the heading "Features" - for my model, no HDMI 1.4 ARC.
For the Series 6 40" LED, ARC is supported. I haven't compared a lot of Samsung models but I suspect that ARC is supported from Series 6 on.