The update from 0.24.x to 0.25.x brings many, many changes and improvements to puppet. The upstream release notes cover them in detail:
http://reductivelabs.com/trac/puppet/wiki/ReleaseNotes
Of note is that 0.25.x clients do not work with 0.24.x masters, so it is important to update the master before or at the same time as clients.
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This update has been pushed to testing
This update has been obsoleted