"I ♥ NY" was part of a campaign to attract tourism, and became a pop-culture symbol; it's not an issue of copyright. In fact, using the heart is more likely to get them in trouble, since it's trademarked by NY and they go against imitators.
http://en.wikipedia.org/wiki/Cafe_Hon#.22Hon.22_as_a_tradema...
It also seems like a lot of amateur t-shirt sites have tons of custom i <3 shirts.