PRISM/CBSD means that a credit card you have been using has been transferred to CitiBank Mastercard or CitiCorp. What Citibank does is notify you through the mail of this transfer, and if you don't call them to indicate you do do not want their card, you automatically receive their Mastercard card in the mail.
Yes, it is a scam. Unfortunately, CitiBank is counting on you to NOT READ your junk mail carefully.
Here's what it says on your Experian report:
New Account xx/xx/07
PRISM/CBSD
PO BOX 6003
HAGERSTOWN MD, 217476003
Banking; New Bank/Credit
I did not receive the card CitiBank claimed they sent me.
I called 1-800-950-5118 to cancel the card, to check to make sure the balance was still at zero, and to report the card as lost or stolen.