It's also likely that the Pads and Rotors weren't properly broken in. A lot of the Infinitis' make that kind of a sound, and in most cases the technicians aren't breaking them in properly.
Do this.
First feel your rotors for bumps. Since they're new, it shouldn't have any. if there are, take it back to Lexus and tell them the brakes are squeeking. I'm hoping they'll replace the rotors under warrantly.
If they do,
-Drive your car at 20mph for 1 minute, and depress the brakes to a complete stop.
-put it in Park for a 2 minutes (cool down)
--Drive your car at 50mph for 3minutes, and depress the brakes...
-Put it in Park for 15 minutes (cool down).
---Drive your car at 75 mph for 15 minutes, depress the brakes...
-Put it in park for 20 minutes.
This should break the pads and rotors in correctly.