According to the documents, the update will be distributed in six separate waves between May 19 and May 30 "to ensure a high quality deployment." Additionally, while not technically part of the automatic update, Avatar Kinect is listed for a May 27 arrival as a separate download.
Aside from offering support for the new Microsoft disc format -- which will "be used in future game titles releasing later in 2011" -- the spring system update will enable PayPal support on the console, extending the payment option "to new international markets." PayPal will be "the only user facing feature" of the update.
Update: Microsoft tells us, "We have nothing to announce."
Update 2: May 27 has come and gone, and alas, Avatar Kinect has yet to launch. PayPal was added, however, on May 19.