Quote:Well, it can arise from the Colorado gambit as well...

Anyway, 3...d6 would probably transpose to the Leningrad after 4.d5 Nb8 and ...Ne5 may also be playable.
Yes, but 1.e4 Nc6 2.Nf3 f5 3.d3? (3.e5!) 3...e5 is quite an unusual opening
As for 1.Nf3 f5 2.d3 Nc6 3.d4 d6 4.d5 Nb8 it seems to me that the maneuvre d2-d3-d4-d5 has taken one move more than the usual d2-d4-d5. On the other hand Nb8-c6-b8 takes two moves more than the usual zero. A quick calculation leaves White a clear tempo up should Black choose to play a Leningrad.
After 4...Ne5 5.Nxe5 dxe5 Black gets doubled e-pawns. I like these if I can later push f5-f4 against a White g3 pawn, since then the e5-pawn has some use (i.e. in the 8...Ne5 main line Leningrad).
I am not so sure that the position after 5...dxe5 6.e4 (or first 6.Nc3 and then e2-e4) is so promising for Black.
It's similar to the Krause variation in the Dutch. There was an article in Kaissiber some years ago, but here it looks like White has a head-start (despite the tempo-loss) in harassing Nc6.
Apart from the fact that I liked MNb's analysis, Black can also (I know I have) contemplate the move-order 1.Nf3 d6. Then 2.d4 f5 is a regular Dutch, while 2.b3 f5 transposes to 1.Nf3 f5 2.b3 d6! (threat e7-e5) which I like for Black.
The only bump in the road is 2.e4 which is no longer a gambit, but being a Sicilian Dragon player I would simply reply 2...c5 or perhaps 2...g6 and 3...Bg7/4...c5 or 3...Nf6 if White tries 3.c4 this avoid the Maroczy in favour of a King's Indian (but one where White has been denied the Sämisch, the Four pawns and several other annoying systems).
A third possibility is 1.Nf3 g6 keeping the option ...d7-d5 in one go. This is useful for Grünfeld players, while 2.e4 c5 is a hyperaccelerated Dragon.
In fact there are tons of ways of tricking White into unknown waters. E.g.: 1.d4 g6 2.e4 Bg7 3.Nf3 c5!? (or alternatively 1.Nf3 g6 2.e4 Bg7 3.d4 c5) but this also demands that Black can play the Ben-Oni.
I guess Black is spoilt for choice here, depending on which other openings one likes. Especially the Sicilian Dragon connection should appeal to Leningrad players. I think the Leningrad Dutch could also justifyably be called the Dutch Dragon

Perhaps more players would try it then...