Pretty interesting stuff. I wasn't aware of this before you posted this.

Here are more details:
Introducing MakerBot® Water Soluble PVA 1.75mm / 1.8mm filament!

This polyvinyl alcohol plastic comes in our custom extruded 1.75mm / 1.8mm filament size. You can just swap the filament from your extruder, and load in the PVA plastic, and then dissolve your parts in water. This plastic builds parts just like ABS, or PLA, and the prints come out a matte white color. It's one kilogram on our heavy duty spool, and ships vacuum sealed with desiccant.

We call this filament an "experimental" plastic, because your extrusion temperatures will be different (160-175C) from standard ABS settings, and you should make sure not to bring it to ABS melting point temperatures. Please read the below instructions and adjust your machine prior to use. There may be some tweaking of your settings for optimal output. When complete, the parts can then be dropped into cold water to dissolve.

Make sure to read the usage instructions and warnings on the MakerBot Water Soluble PVA Wiki page

MakerBot have come up with a few uses for water soluble components at the BotCave, and the possibilities are amazing! Some of the applications include: making soluble molds, building removable support structures, biotechnology applications, or fascinating science experiments of your own creation. We love introducing products like this, because inevitably, the best uses will come from you! Send us your ideas and your results, we'd love to see them! This plastic is off white- and smells a bit like white craft glue.
The science behind MakerBot® Water-Soluble Polyvinyl alcohol:

• Melt flow index (190 oC, 2.16kg); 1.5 – 3.5 g/ 10min
• Melting point; 160 – 170C
• Glass Transition Temperature; 45 – 55C
• Specific Heat; 0.4 cal/ gC
• Density; 1.25 – 1.35 g/cm3
• Solubility into water; soluble into cold water
I would think this would make more sense for those people who print out molds that they want dissolved, unless I'm totally missing the point of this stuff.