A13: It was only quite recently (2002) that I became aware of the possibility of playing this way as it wasn't mentioned in any of my text books when I was learning the English (probably because White would be expected to play d4 and transpose into a Slav).
I think Black players who like to play Meran-type Slav positions are quite likely to play like this, thinking that White will continue with d4 at some point so that they can get the position they want.
I remember someone asking about this when I was in charge of the Flank Openings site, and soon after someone actually played something similar against me - have a look at the 'Réti - 4...Nbd7' game and my game against Randisi (either from ChessPub.exe or the PGN Games Archive).
A10: Well, I couldn't cover every conceivable move, and 1...b5?, which loses a pawn for nothing, would certainly come into the category of 'lines that don't deserve any space'!
Curiously, the strongest player to play this is my Schott Mainz team-mate in Germany, Tilmann Vogler, so I will have to ask him about it sometime! However, the idea is to answer 2 cxb5 with 2...a6 and gambit a pawn for a couple of open lines on the queenside. The problem is that after 3 bxa6 Bxa6 White hasn't committed his centre as in the Benko Gambit and can continue, say, 4 Nf3, 5 g3, 6 Bg2, 7 0-0 before deciding on his central formation.