As far as I know, after 9..Ne5, 10.Nb3 is indeed played to clear d4 for the bishop.  It's thought to discourage ..d5 because of the line 10..d5 11.Bd4 0-0 12.Kb1 dxe4 13.Qf4 Bd6 14.Nxe4 Nxe4 15.Qxe4 when White has built up an attacking position with little risk to himself.  This was played in Movsesian-Polgar, Budapest 2003.  Black has to retreat the Knight because 15..Bd7 is now met by 16.Bxe5 Bxe5 17.Rxd7 Qxd7 18.Bd3 g6 19.Qxe5 +-.  Polgar chose 15..Nc6 (15..Ng6 has also been tried) and after 16.Bc3 Bd7 (16..f5) 17.g4 Rfe8 18.Bd3 g6 19.Rhe1 (preventing ..Bd6-e5) 19..e5 20.h4 Ne7 21.Bc4 Bc6 22.Qd3 Rad8 23.Qe3 Nc8 24.h5 Nb6 25.Na5 Na4 26.Bd2 Bc5 27.Qe2 Bd5 28.hxg6 hxg6 was lucky not to lose.  Movsesian drifted here with 29.Bxd5?! instead of the better 29.Qh2 or 29.Bb3.

11. Qe1 is a good move because it takes advantage of the differences between the "English Attack" Taimanov and the analogous lines of the Najdorf; Black has different tactical problems.

To my mind, as a Taimanov player, the whole point of being alive in the "English Attack" lines is to play d7-d5 in one move; generally speaking relative to the Najdorf you get Qd8-c7 for free.

Unfortunately it isn't entirely clear that's a good thing.

Could you provide sample lines? I haven't seen 10.Nxc6 before and my first impression is that it reinforces Black's centre after 10...bxc6.
A sample line of mine illustrating this is 10.Nxc6 bxc6 11.g4 d5 12.g5 Nd7 13.h4 Rb8 14.h5 e5 15.exd5 cxd5 16.Qxd5 Bxc3 17.bxc3 Qxc3 and White's King enjoys too much fresh air.

Not a stupid question at all - it took the world's best some years to understand the strength of 11.Qe1. 

The most obvious idea is that Nxb5 is threatened, and White now can swing the queen to g3 where it puts pressure on both the kingside and the black pieces conglomerated on the b8-h2 diagonal. After 11...Be7, the main line runs 12.f4 (this is stronger with the queen on e1 than on d2) Ng6 13.e5 Ng4 and now both 14.Ne4 and 14.Bd2 are currently considered a little better for White. Note that the queen on e1 can be useful to defend a knight on e4 without blocking the d-file (as is the case after Bd3). 

If you play through Karjakin's games in this line, you should pick up the key ideas quite quickly.

Thanks for your help guys! Nice games, I will analyse the opening and the games

'11.Qd4...doesn't look very ambitious after 11...Nc6 12.Qb6 Qb6 13.Bb6' - De La Villa.

It's been known for a couple of years now that only 11.Qe1! secures an advantage for White. Vachier Lagrave-Vitiugov, Plovdiv 2008 and Karjakin-Stellwagen, Wijk Aan Zee 2009 are two key games in this line. 

For the record, 10...d5!? isn't bad either.