I have a 2005 Specialized HardRock Sport with disc brakes.
When I'm slowing down from a reasonable speed with the front brake, I noticed that the top of the front wheel appears to deflect sideways toward the disc. I can't tell when I'm riding but I suspect that the whole wheel is twisting. This doesn't seem to be safe to me, and I'm don't remember that happening before when I ride.
I tightened the QR on the front already. What else could be the cause of this or where should I start to look? Or has it always done this and I never noticed before?
I'd take a close look at your dropout and your QR skewer. The rotating disc exerts an upward force on the disc caliper, which tends to want to push the dropout up and away from the axle (or as normally stated, pushes the axle down and out of the dropout). Some calculations at Jules & James' comprehensive site computes the force generated to be as much as 3800N.
To achieve a 6mm offset at the top of the tire you'd need the axle to move about 1mm within the dropout. Try spinning the wheel in a bike stand with the skewer loose enough to allow free sliding but not wheel ejection, and see if you can replicate the same effect you're seeing out in the real world.
I suspect it's the fork that's deforming and causing the "twist". When you brake with disk brakes, enormous force is placed on one side of the fork and essentially none on the other. The fork is going to deform -- it's just a question of how much.
Caliper brakes, of course, distribute the force evenly, so you don't notice any deformation, even though there is some from the wheel pushing backwards on the fork.