jUpgrade

Upgrade is a simple Joomla 1.5 component that will download a version of Joomla 1.7, decompress it and install it. After that, it should migrate all of the data from old Joomla 1.5 to the new Joomla 1.7 installation.

http://extensions.joomla.org/extensions/migration-a-conversion/joomla-migration/11658