Not sure where to post this (feel free to move Mr. Mod) but thought I'd share.
I have a $50 amazon.com coupon due to arrive in the mail and inquired as to whether it can be used on amazonwireless.com or not. Here is the response I got. (Disclaimer: Keep in mind this is the current policy and they often, and do, change).
"Amazon gift cards can only be redeemed at the issuing website. On AmazonWireless, you shop securely with your Amazon.com account, and you benefit from Amazon's world-class service and security; however, AmazonWireless operates a stand-alone website independent of Amazon.com"
Sure enough, there is no place to put an amazon.com gift certificate code in during checkout. I did get an email back in less than 24 hours after asking a question.