Ian Maclennan announced on Joomla! Bug Squad Google group that Joomla! 1.5.19 will be ready tomorrow and it will include an upgrade plugin to move from MooTools 1.1 to 1.2

Administrators will be able to choose weather to upgrade MooTools or not. At the moment most of the extensions developed are based on the old version of MooTools. If you're willing to continue supporting 1.5 you should start migrating your extensions to the new version. Lastly we should thank the Bug Squad team for their amazing work!