I've been seeing this more and more, and in a desperate attempt to find out what it means I came across the following (only checked one site):
It signifies a heart apparently. (...?)
So basically a lot of people have been telling a lot of other people that they love them.
Now, this is all nice and anti-war and flower-power and what not, but that's just too darn woosie and jiggly for my liking. Seriously.
So I now officially proclaim that when I see <3 anywhere, I'll refuse to look at it side-ways from left to right (turning the "heart" upright), and will instead and hence forth look at it sideways from right to left (effectively turning the "heart" upside down).
The point? Well, if you look at it that way it resembles something more akin to cleavage, which just makes more sense at the end of the day when you think about it.
EDIT: Like for instance if I was to type "its <3" I'd in fact be saying "its the tits!", as opposed to "its love baby!".