I mentioned this on Twitter and wanted to mention it here ...
It's possible that the strange distribution of very dark and very light on Pluto is a result of the kind of thermal runaway effect that has made Saturn's moon Iapetus, shown here, look the way it does. Regions that began life as slightly darker, and/or receive more direct sunlight, will grow even more so over time with the vaporization, and hence removal, of ice ... a process we know is happening on Pluto.
This effect can work really well if the ice is not pure ice but has dark impurities in it, as older Pluto ice most certainly does, and there are differences in slopes that result in differential solar heating, and the body is a slow rotater. All conditions are likely present at Pluto.
If some vaporized material remains bound to Pluto, it will eventually condense on the coldest place on the body, making that region brighter. If this happens over billions of years, it could result in a large ice deposit. Those colder locations may be, at this moment, the 'heart', which is on the anti-Charon facing hemisphere and is not receiving what little heat there may be to receive from Charon, as well as the un-solar-illuminated hemisphere of Pluto, which we haven't seen yet, and regions that are not so directly facing the sun.
Science on the fly ... as it happens and maybe totally wrong! We shall see.
Links to Iapetus picture captions:
Hi Res mostly-black view: http://www.ciclops.org/view.php?id=3764
Hi Res mostly-white view: http://www.ciclops.org/view.php?id=3786
Full body view: http://www.ciclops.org/view.php?id=3787