WordPress v1.5.1 has been released. This new release fixes over 170 bug fixes, adds many enhancements, and contains an important security fix. The release notes are available here. Comprehensive upgrade instructions (from v1.5 to v1.5.1), along with a thread discussing issues that may pop up with the upgrade procedures, can be found here. If you are upgrading from WordPress v1.2.x to WordPress v1.5.1, please follow this guide.
Note: If you are using KafkaesquÃ’s Entity2NCR, disable it before upgrading. This plugin’s functionality has been included with WordPress v1.5.1 and will cause errors if it is active during the upgrade process.