1 d4 d5 2 c4 c6 3 cxd5 Nf6
If white wants a boring game and tries to play the Slav exchange, then black could transpose to your idea, nightwatchman. It must have some surprise value. I like the way it becomes similar to a Budapest. In the end, however, white is just a pawn up.
Huh. I always regarded the Marshall defence as a weak one...
Well, actually, it is weak. It's the Slav portion that's backing it up.
11.h3 is White's best bet, to fend off the knight. It could be done, but black can pull out his newly castled rook. All in all, it's a bad day for White.
This requires more analysis.
Sorry to double post, but what about 6.Bg5?
This requires more analysis.
Something tells me it's already been done (years ago)...
Huh. I always regarded the Marshall defence as a weak one...
Well, actually, it is weak. It's the Slav portion that's backing it up.
11.h3 is White's best bet, to fend off the knight. It could be done, but black can pull out his newly castled rook. All in all, it's a bad day for White.
This requires more analysis.
How can you "always have regarded the Marshall as weak" when you've only been playing for a month?
There's no need to be so rude. I've been playing chess for barely a month.
Whatever. If you say it sucks, it does.
Okay, I'll just stick to my book openings.
Always does not imply a long span of time.
If I said to you,"John has always been crying", it does not mean that John is crying repeatedly. It means he has been crying for his whole life. Which may be 2 seconds.
In this case, "always" refers to that one month since I started playing chess. I learnt the rules about three years before, but my first match was a month ago. I say "always" because I have both played and seen people playing the Marshall, with poor results.
Aptly enough, gd, I guess this whole thing must be akin to the concept of "dog years"... 
It sure struck me as odd. When I was still umbilical cord new to chess I wasn't regarding the soundness of recondite openings. More like trying to figure out what to do in the Four Knights setup beginners play into.
I was still taking 2 steps with the horse and then 1 over, the way my dad showed me. Oh yeah, and then whenever there was a mate (or a purported one at least) we had to try all the squares in the king's field to make sure.
All I really wanted was that $5 he promised me the first time I won (I finally got it too!...after a month).
Then I went out and bought a watch with it, only it turned out my stupid wrist was allergic to the steel backing so I couldn't wear it. 
It sure struck me as odd. When I was still umbilical cord new to chess I wasn't regarding the soundness of recondite openings. More like trying to figure out what to do in the Four Knights setup beginners play into.
I never got into Four Knights for some reason, and one of the most emphasized pieces of advice I was given was "Learn your openings". I went around asking which openings to learn, got the answer "Queen's Gambit", and went and did some reasearch on Wikipedia.
When I faced it, I played the Marshall Defence and lost. Then I went on Wikipedia again and looked at the Marshall Defence and concluded it wasn't good.
Just wait'll you get Fried Liver fever...my high school club was abuzz with that one for months.
I'm sure that most people are aware that the Marshall Defence (1. d4 d5 2. c4 Nf6?) is generally considered unsound due to (3. cxd5 Nxd5 4. Nf3). Today though, I was playing against Houdini and it entered the Marshall Defence (it wasn't using an opening book), however the game did not follow the usual line instead proceeding (3. cxd5 c6!?). This move is actually quite interesting and I went over some of the possibilities.
White can simply ignore the pawn and continue either 4. Nf3 or 4. Nc3, at which point 4...cxd5 gives black a fairly comfortable game as white will not be able to play the intended e4.
If white presses for the advantage and takes the pawn on c6 the game continues (4. dxc6 Nxc6 5. Nf3 e5!?), with the following position...
Black offers a second pawn which white can take in with either (6. dxe5 or 6. Nxe5) either move leads to a Queen trade on d1 (6. dxe5 Qxd1 or 6. Nxe5 Nxe5 7. dxe5 Qxd1) followed by the forced sequence (8. Kxd1 Ng4 threatening to fork the king and rook 9. Ke1 Bc5 10. e3 OO). Now white's king is stuck in the centre and black has a big lead in development, even though the Queen's are off he should still have some attacking chances. The game up to this point looked like this...
Black will win black at least one of the pawns easily. If white can defend accurately he will probably have an advantage, but that's never stopped attacking players from trying these sorts of things.
Perhaps better for white was (6. e3), although in this line black still obtains a lead in development with Queen's on so there is an element of risk.
Personally I doubt I will ever play this as black except in blitz, but nevertheless I thought it might interest more attacking players than myself.