This just in from my E* Insider:
So, for those that haven't gotten yellow cards for their 721's, heads up.Just a heads up--the smartcard exchange for the 721 has had a few issues.
If and when they send you the yellow card for your 721, do not try to
activate the card. The activations have not been successful. Instead, call
DISH and tell them you have a 721. They will express ship a replacement
receiver for free instead of activating the card.