On: Nasa just emailed a wrench to the ISS

are there design considerations for 3d printing in low gravity? It seems like not being able to assume gravity would be a significant hurdle.

Yes. Apparently they use materials with a lower melting point than those commonly found in Makerspaces:

http://www.nasa.gov/mission_pages/station/research/experimen...

I believe this is the first time they have used this technology outside of experimentation.