The phonemes /d/ and /t/ are both alveolar stops that are only differentiated in their voicing. Because of the close relationship between the /d/ and /t/ phonemes, when they are adjacent to each other they blend and become one sound. The result of this is that when read aloud "iced to meet you" sounds an awful lot like "nice to meet you." This coincidence was clearly a large part of McDonald's intended, though somewhat covert, message on the billboard - McDonald's iced coffee is so good the consumer will think it nice to have met.
Because the /d/ and /t/ phonemes can not be distinguished verbally, this campaign only works in print as it is the act of reading the billboard and reciting it aloud to yourself that makes it fun and gets the intended message across.