Sony recently announced and then promptly put on hold a software development program for networked devices called SNAP. Within hours, it seems the project was put on hold. Everyone seems to be reporting that the SDK is no longer available for download. It may not be linked from the main page anymore, but a quick search was all it took to find the SDK download page, which still seems to be working here.

EDIT: that page no longer has the SDK after all. I have a copy, but I don't want to distribute it if I don't have the right to. Instead, I did publish some of the LGPL'ed code on github here.