I suspect it is just that, warped rotors. Just because the fronts are new doesn't mean anything. Plus you can get the pulsing from the rears even if the fronts are fine. I have mild pulsing also.
I've had two other "H" body cars ('92 Buick LeSabre & '94 Olds Eighty Eight) and I had to pay close attention to the accuracy of the rotors. Since I have a lathe, I turned them myself and made sure they were within .0005" parallel and the same runout. This helped a lot, but then after a few heat cycles I could tell something had moved a little. It is probably due to the material the rotors are made of. It wouldn't surprise me if the 2000+ rotors are the same way.
2001 Bonneville SE - Dark Bronzemist Metallic - Mods: 180* thermostat, iPod adapter, Digital OnStar Upgrade
Planned Audio System upgrades: Front & Rear 7 Band Equalizer, Amps, Hands Free Bluetooth Cell Phone kit
Planned mods: Front PEM, Ported LIM, Ported Heads, ZZP SR Rockers, Aluminum Gaskets & Reprogram PCM