Here's the email they send out regarding these:
Thank you for contacting Universal Studios Home Entertainment. We strive to provide the highest standard in product quality and customer care.
We would like to send you a prepaid envelope in which to return your "Children of Men" HD-DVD for evaluation and replacement. Please let us know:
1. Your daytime telephone number (without call blocking)-
2. Shipping address (not P.O. Box) -
3. Manufacturer and model # of your player -
4. Retailer where you purchased the HD-DVD -
Once you have returned the disc, please allow 3 - 6 weeks to receive your replacement.
My copy had the magnetic security tag glued onto the SDDVD side of the disc when I received it, so I emailed them about it and got the above response. Never bothered sending it in as the HDDVD side seemed to play fine and I wouldn't like to go through the hassles of their exchange process.
Also, unless it has changed recently, they're not too good about responding to emails (I finally got the above response after my 2nd email a while after my first didn't get an answer)