It's called "canvas fingerprinting." It started being tested earlier this year, and it was discovered by Princeton University and KU Leuven University in Belgium. And it's being used by sites ranging from WhiteHouse.gov to YouPorn.
It works like this -- web sites, instead of giving your browser a cookie to track you around the Internet with, will instead create a hidden, unique image (hence the term "canvas fingerprinting") that can be tracked around the web to deliver better targeted ads or worse (you just know the black hats are going to have a field day with this). AdBlock Plus is ineffective, as are incognito modes on your browser.
The program was developed by AddThis. They claim the data so far has only been used internally. Yeah, we'll see how long that lasts. AddThis offers an "opt-out" cookie, and we know how well those things work. YouPorn discovered it had been included by AddThis to their site without their permission. YouPorn has dumped any and all AddThis tech from their site.
To fight this thing, you're going to have to jump through some hoops and live with some inconveniences. Gizmodo is suggesting using Tor, my personal favorite Firefox add-on NoScript (which you may remember from the FireSheep debacle), use the Chameleon browser, or just block scripting all together. This will limit your web experience somewhat, but there's nothing else that can be done.
This is in a paper published by two prestigious universities. The black hats have already gotten a hold of it.
But the white hats have, too.
I expect the best minds in Open Source to begin working on this little situation, and I'm sure they'll come up with something. Until then, stay frosty, people.