From the JSTOR news website:
We are excited to announce that the new JSTOR platform will be launched on Friday,JSTOR and over 230 research databases and indexes are available via the Library's Articles, Databases link.
April 4. The behind-the-scenes process for switching from the existing JSTOR to the new platform will begin around 7:00 a.m. EST.
A document describing features of the new platform is included in the JSTOR Sandbox http://sandbox.jstor.org/. The Sandbox also has links to tutorials and training materials focused on the new JSTOR interface.