While I never provide specific answers without a full and proper legal consultation and a full review.
This is a great question.
Off the top of my head, the answer I think is "yes" but you would not be trademarking the heart - you'd be trademarking the look of the phrase with your artistically rendered version. I didn't read extensively on this issue but the articles I'm reading point to some famous "I Heart NY" or "I ♥ NY" trademark infringement lawsuit - a mark owned by the state of New York.
This article in Business Week discusses something along the lines of differences of appearance concerning a lawsuit for an "I ♥ NC" lawsuit and also references the "I ♥ Yoga" lawsuit where New York State shut down a Bikram Yoga based effort that used the instantly recognizable heart.
There is more about it in Wikipedia as well, even though it's not always the most reliable source.
So my gut feeling here is that if you design a shirt that doesn't make people instantly think of the famous black typewriter text, big red heart "I heart NY" t-shirt and that there might be an association, you might be fine with a nice blue Arial font and tye dye colored heart.