"There will be a platform release this Saturday night May 17, 2008, from 9-10:30 p.m. Pacific. During this window, Ning.com and all networks will be offline.
In addition to some behind-the-scenes work to improve scalability and performance across the Ning Platform, this release will include changes to the design of Ning.com."
Another downtime is "scheduled for Monday night (5/19/08) from 9:00-9:15pm Pacific."
The changes have to do with network admins and creating new networks. As users, we won't notice much, if any, change. For those curious about the details