Do I need to "fill in" concrete control joints before floating floor installation?
As far as the joint is concerned, you could fill it with a self leveling sealant, as represton says.  I don't think it will affect the flooring either way, though, as your moisture barrier will keep the humidity away from the flooring (and the room).

And, a point of clarity...  This is not an expansion joint, this is a crack control joint.  It is intentionally weaker so that as the slabs settle, as they eventually will, the cracks will occur along the joint and be less obtrusive.

But more important for all of this, I feel, is whether or not the slab is flat enough for an unsecured laminate.  High spots will allow the flooring to move, resulting either in an uneven feel under foot or noise.  You might want to caulk in the joints to keep the leveling compound out, then apply a self leveling cement based product to the whole floor.  Have you run a straight edge around to see if that slab is nice and flat, and will the flooring package insert tell you what the tolerances are?