WWL-TV had reported that several t-shirt shop owners had been told [by the NFL] to stop selling the shirts because the NFL owns the rights to "Who Dat" on any merchandise that might try to tie the phrase with the Saints.
Greg Aiello is quoted: "If Who Dat is used in a manner to refer to Saints football, we own trademark rights to it. We get involved when there is use of the Saints logo or “Who Dat” used in combination with Saints logos or football imagery."
The WWL report gives some information relevant to any upcoming legal fight: As for Nuccio and his trademark, it apparently expired back in 1993, according to records from the Louisiana Secretary of State's Office. Someone did register the term Who Dat three years ago. The group: New Orleans Louisiana Saints, L.L.C.