I just applied for this card on Friday. I got that same message, but I also got a reference number further down the page and it said I could check back for a decision using that number, but I could only check once. I checked the next day and found out I was approved. Maybe call back, especially if it was just today, and ask if you could have that number to check online, or maybe they can give you the decision then. The CRS saying you'll get a letter could also just mean that she/he couldn't see a decision. If you were denied and CSR could see that, I don't see why they wouldn't have told you. Hope it's not a denial. Good Luck.
11/2013 FICO: 517 (TU), 560 (EX), 539 (EQ)
11/2013 FAKO: 593 (TU), 591 (EX), 595 (EQ)