Omg embarrassing story time!
I used to think that was like a really happy cat. 'Cause you know the "evil cat" smilie? (>:3) It's sometimes shortened to >3. So when I first saw <3, I thought it was just the opposite, lmao. ;_;' It took me a really long time to realize it was supposed to be a heart turned sideways. Luckily the context I always saw/used the smilie in was exactly the same as a heart but... still. XD;
Anyway, I don't use it much. Usually only on MSN when I can't do the alt code for ♥, which is the superior heart.