Development of WordPress moves fairly quickly and day-to-day things break as often as they are fixed. This high churn is part of our development process that aims to produce the most stable releases possible.
You can track changes on the web or through our SVN mailing list but many people choose to run the nightly builds to help us identify bugs. If you would like to be part of this process, please join the WP Testers mailing list and download the latest nightly build.
WP's automatic upgrade can be used to automatically upgrade to betas and nightly builds for the development branch or the latest stable branch. To get onto a development upgrade path, install the WordPress Beta Tester plugin.