Apparently, Joomla 1.5.16 had a couple of bugs in it. And those bugs were serious enough to release another version now.
You can upgrade directly from Joomla 1.5.15 (or an older version) to Joomla 1.5.17.
The announcement reads:
The Joomla Project announces the immediate availability of Joomla 1.5.17 [Wojmamni ama woobusani]. This is a priority release to correct two issues in version 1.5.16. Although there are no security issues fixed in this release, we consider it a security release because a security-related bug has been fixed and because many sites may be upgraded directly from 1.5.15 to 1.5.17.
- Check the Joomla 1.5.17 Post-Release FAQs to see if there are important items and helpful hints discovered after the release.
- If you have modified core template overrides, please be sure to back them up before upgrading.
Fixes in Joomla 1.5.17
- Updated ru-RU installation language (20239)
- Added en-AU installation language (20220)
- Updated help sites list (20238)
- Fixed problem logging in when Session Handler is set to None (20221)
- Fixed error message when running Joomla! in a PHP version prior to version 5.2 (20219)
- Reverted change to JFolder::makesafe method that introduced a bug (16506)