It looks like Sony thinks the Credit was a good deal for them too. If you have a look at their dedicated
Reader Page, under item #9, Promotions (I can't link to it directly, 'cause it's a Flash element), they're offering the $50 credit again for the next 2 months (March & April). So if you haven't authorized your Reader to an Account yet, wait 'til tomorrow (03/01) and you can get the credit.
The same page is also touting the $100 credit for opening a Sony Credit Card account, so apparently they aren't
too upset by the idea of it being used to defray part of the Reader's cost.