Seems like this is an ongoing issue for some people with this promotion as I am also having troubles receiving my promotional rewards for joining XBL Rewards.
On Feb. 24/12, I received an email to join XBL Rewards. To my knowledge, I had not joined this program in the past as I received confirmation later that week upon registering for the program. The promotion was for 800 free MS points for applicable residents (ie. Canada for myself). Contacted XBL support and on the online chat, Alejandro directed me to post in Section 7 of the Xbox forums. It has now been 6 weeks since joining and no information yet on the points being updated to my account as per Microsoft's Billing History online.
Can someone from XBL look into this for me?