1. Nf3 d5 2. c4 d4

Any comments?

Reminds me of this thread:

http://www.chess.com/forum/view/chess-openings/1-d4-nf6-2-d5

Probably worth a look-see there to get some ideas.

 Thanks for the link.

Thanks for your comments! I did consider that they might not take the pawn but didn't know what to do.

2...d4 is a sharp move, risky for both sides, and might be considered the acid test of the Reti opening. B grabs space and challenge W to prove he can generate enough play. The position will likely become a sort of reversed benoni with an extra tempo, with very interesting and sharp play. 3.e3 is a very reasonable move in this structure, as well as the more flexible 3.g3, planning d3 and e3 later. 3...dxe3? is a very bad positional blunder for black, W enjoying a much better game after 4.fxe3 thanks to his powerful centre. Few players will play this, protecting the pawn is the natural reaction.

 Thanks for your insight. So in the resulting position, what should I do? Capture?

exd4 doesn't feel right, and it makes the future black d4 pawn strong. b4 might be worth some thought.

a move sooner, like 3.b4 followed by 4.Bb2 looks strong

Gambiting a pawn with b4 is interesing,but the more usual thing is to continue with g3, Bg2, 0-0, d3,Re1, and indeed exd4, in  some order. exd4 immediately is ok, followed by developing with the setup i mentioned. Then your chances are based on the pressure against black e-pawn, the powerful fianchettoed bishop, and possibly a queenside expansion with b4. Frankly speaking this opening is not what i would suggest to a player rated under 2000, because of its complex strategical feathures, but you can of course give it a try.

Yes, I think so.

ok.

3b4 is what Delchev mostly looks at in recent book on Reti.

I added a diagram for 3. b4.