Please include the following in the 2.6 release notes:

At the request of many plugin authors, we upgraded joda-time from 0.98 to 1.4 in Confluence 2.6. This is a binary-incompatible change, meaning all plugins that use joda-time's date formatting and parsing classes will need to be recompiled against Confluence 2.6. Plugins that use joda-time's date formatting and parsing classes that were compiled against Confluence 2.5 or earlier will not work in Confluence 2.6.

Details on the affected functionality are in joda-time's upgrade notes: http://joda-time.sourceforge.net/upgradeto099.html