Do you want the look at vector in world space? If so, assuming you have a left handed coordinate system you can do the following:

In pseudo-code:

Matrix viewMatrix = camera->getViewMatrix();
viewMatrix.invert();
Vector3 lookAtInViewSpace(0, 0, 1); // Forward vector (use 0, 0, -1 if you have a right-handed coordinate system)
Vector3 lookAtInWorldSpace = lookAtInViewSpace * viewMatrix; // Transform into world space.

I am not that familiar with D3DX (which you seem to be using) but I think the transformation function you want is D3DXVec3TransformCoord.

UPDATE: No sorry, I was wrong. It is D3DXVec3TransformNormal, since you are transforming a direction, not a point.

Do you want the look at vector in world space? If so, assuming you have a left handed coordinate system you can do the following:

In pseudo-code:

Matrix viewMatrix = camera->getViewMatrix();
viewMatrix.invert();
Vector3 lookAtInViewSpace(0, 0, 1); // Forward vector (use 0, 0, -1 if you have a right-handed coordinate system)
Vector3 lookAtInWorldSpace = lookAtInViewSpace * viewMatrix; // Transform into world space.

I am not that familiar with D3DX (which you seem to be using) but I think the transformation function you want is D3DXVec3TransformCoord.

UPDATE: No sorry, I was wrong. It is D3DXVec3TransformNormal, since you are transforming a direction, not a point.

Do you want the look at vector in world space? If so, assuming you have a left handed coordinate system you can do the following:

In pseudo-code:

Matrix viewMatrix = camera->getViewMatrix();
viewMatrix.invert();
Vector3 lookAtInViewSpace(0, 0, 1); // Forward vector
Vector3 lookAtInWorldSpace = lookAtInViewSpace * viewMatrix; // Transform into world space.

I am not that familiar with D3DX (which you seem to be using) but I think the transformation function you want is D3DXVec3TransformCoord.