"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.
I worried a little when I started selling this shirt: http://mrcoles.com/media/img/cool-harold-i-ny-ny-fractal-shi... but I figured it was either different enough or could be applied as a parody? (Or probably just no one cares, since it's not popular enough…)
It also seems like a lot of amateur t-shirt sites have tons of custom i <3 shirts.
http://en.wikipedia.org/wiki/I_Love_New_York