The cab driver is familiar with both the sayings from Blue's Clues, and the Blue's Clues theme song. Despite Steve saying "It's nice to meet you", the cab driver says he feels like he already knows them. Steve and Joe do not disagree, and also seem familiar with the cab driver, even if they are not certain why.
What was this in reference to? Why did he feel he already knew Steve and Joe? It's not mentioned again in the movie, and my searching so far reveals no previous association between Alex Winter and Blue's Clues (except for a Robot Chicken parody which I highly doubt is the intended reference).