(Disclosure, I may receive a small referral fee when you make a purchase through links on this post.)
I was one of the people who purchased the Google Chromecast on launch day and ordered it on Amazon shortly after. That was 4 days ago. Today, I received this in my email which has a link to redeem my Netflix code. For those who bought their Chromecast via Amazon, you should be getting them today if you are one of the lucky ones who got it early. I would assume the ones you bought on Google Play store will follow shortly after you receive your item. If you bought it at Best Buy store, it should be on your receipt.
Once you get the code, follow these instruction to redeem!