This is the message they sent out:
We have recently become aware that certain versions of Mac Preview are incompatible with the newest generation of JSTOR PDFs. For some users, this means that only the cover page is displaying. Other users have reported that the downloaded article appears to be missing the first portion of the article—it begins somewhere after the first 10 pages of the article. The article is in fact intact and complete, but some pages are not displayed properly. We are working to fix this and plan to continue to support the use of Preview with JSTOR PDFs. We apologize for the problems that this is causing for Mac users.
For now, the quickest workaround is to download the latest free copy of Adobe Reader, available on the Adobe web site:
Once Adobe Reader is downloaded, please check to make sure that the default PDF viewer is set to Adobe Reader rather than the original PDF viewer installed on your Mac.
The default viewer for PDF documents in Mac OS X is Preview. To change the file association for PDF files:
Select any PDF file and choose File > Get Info.
Choose the application that you want to open PDF files from the Open With menu.
Click Change All, and then click Continue.
You will also want to make sure that the popup blocker is turned off for the JSTOR site. The new PDF files are set to open in a new window and popup blockers may affect this.