The National Association of Science Writers (NASW). A great resource for newbies as well as veterans of the field. I got my foot in the door by joining this organization and attending their annual internship fair.

Council for the Advancement of Science Writing (CASW).

MIT Graduate Program in Science Writing - Massachusetts Institute of Technology, Cambridge, MA.

UC Santa Cruz, Science Communication Program - University of California, Santa Cruz, CA.

Johns Hopkins University, Master of Arts in Science Writing - Johns Hopkins U, Baltimore, MD.

Santa Fe Science Writing Workshop - held every May in Santa Fe, NM.

There are more resources available on each of the sites I've listed that cover topics like graduate program entry requirements, job opportunities, freelance issues, etc.