I remember seeing this idea and being a bit sceptical too, so I've never played it. There's a bit on it in Labollita--Leitao on ChessPublishing which might be worth a look.

Thanks for your reply Michael!
So far I have followed Davies' recommendation of 3.g3 and was looking for some "not so solid"   alternatives...
3.e3 Nc6 4.b4!? & 3.b4!? are obviously something I have to look into.

Anyone knows what Dunnington recommends for white after 1.Nf3 d5 2.c4 d4 in his "Easy Guide to the Reti Opening"?

Hello,

There was a very strange Gelfand v Shirov game, where black played the triangle of pawns, then pumped the a-pawn up the board. At move ten or so had not developed a single peice, but still won!
                 There are new comments on the game in the Gambit book Dynamic Chess, but have not had the chance to look at it yet.

Bye John S

This is alas not entirely true: 1.c4 e6 2.Nc3 d5 3.d4 c6 4.e4 is the Marshall Gambit. This is a bigger disadvantage than 4.cxd5 exd5, when Black has no problems at all developing the queens bishop.
Semi Slav players can avoid this after 1.d4 d5 2.c4 c6 3.Nc3 Nf6 4.Nf3 and then have to play something independent against 1.c4.
But the initial question (1.Nf3) seems to be answered satisfactory.

MNB in the semi-slav black plays 4...e6
your line with 1 nf3 d5 2 c4 c6 3 e3 e6 4...nf6 seems fine for black 
plus after 1 nf3 d5 2 g3 it doesnt make sense for black to play d4 and it seems like i would have to play 2...c6 anyway

Hmm ... well ... it is an option, I suppose, but I am a tad suspicious of this with either colour.

On topic first:
I would like the position after 1.Nf3 d5 2.c4 d4 a lot more if Black were obliged to play ...c5 at some point - then I would indeed feel very happy as White  Unfortunately, he can meet both 3.e3 and 3.g3 with 3...Nc6(!) (which resembles the annoying line 1.d4 Nf6 2.Nf3 c5 3.d5 e6 4.Nc3!?) - that seems pretty equal to me, and not particularly stimulating.

This leaves only 3.b4!? which can get a bit murky after 3...f6 4.e3 e5 5.c5!?  I have no objection to murkiness in principle, but I am not sure about this for White at all. Of course 5.Qb3 is also possible, but Black is OK there.

Having said that, if I played the Slav or Semi-Slav with Black against 1.d4, I would be very sorely tempted to play a Slav set-up against 1.Nf3 and 1.c4, too.  If White sticks to his guns and continues to play a Réti set-up, then the lines with ...Bf5 or ...Bg4 are both pretty reliable weapons for Black.

Of course there are still move order issues: if White plays 1.c4, then after 1...c6 you have to be ready for 2.e4!?, and likewise after 1...Nf6 2.Nc3 c6, 3.e4!? is again an option for White.  It is not an easy matter, to force White down your chosen route!  But that is the nature of things, of course ...

As for MNb's recommendations for Benoni players (moving back off topic   ), believe me, I have thought about this issue!  It is all very well, to play the Symmetrical 1...c5, if you can guarantee that White will subsequently play a quick d2-d4, but if he plays à la Kosten's Dynamic English (i.e. keeping it closed with g3, Bg2, Nc3 and then a3, b4) ... well, this is not to my taste, with Black at least.  Much better imho to go (1.c4) 1...e5 and then either the Reversed Dragon lines or 2...d6 and 3...f5 (MNb - you should approve, this is a Classical Dutch with ...e5 already in!)