Continuation in the sicilian I played a game today in the Sicilian Defence. I was White. I was surprised to see the move 5. ... e5 ! Is this playable in the Sicilian?

[FEN ""]
[StartPly "5"]
[Title "Is 5. ... e5 playable?"]

1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e5


My intuition tells me that this move weakens the d6 square so much and White can develop an attact quite easily. The continuation

 [FEN ""]
[StartPly "5"]
[Title "Is 5. ... e5 playable?"]

1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e5 6. Ndb5


is obvious. White seizes control of the b5 square aiming for the weak d6 square. 

I believe that Black should answer 5. ... e6 taking away the important d5 square from White. Has anyone seen the ... e5 variation before?
This is the Sveshnikov Variation of the Sicilian. It is a fairly popular line even at the highest level.

You are right, that 6. Ndb5 is the critical move. The game usually continues from there: 6. ... d6 7. Bg5 a6 8. Na3 b5

Black has somewhat weakened d5 and the pawn on d6 is weak, but in exchange he has expanded on the queenside and is threatening a fork on b4.