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 you must first make a small change to wp-includes/version.php.
The current release of WP is 2.8. If you peek in the version.php file, you will see this:
$wp_version = '2.8';
If you would like to try out the latest development builds for the upcoming 2.8.1 release, change that line to this:
$wp_version = '2.8.1-dev';
“dev” can be any string. The presence of a suffix on the version tells the automatic upgrade to put you on the upgrade path for 2.8.1 development releases. You will be able to automatically upgrade to each nightly, beta, and RC build for 2.8.1. Once the final 2.8.1 release comes out, automatic upgrade will upgrade you to that official release and put you back on the stable release path. To get back on the 2.8 development branch upgrade path after the release of 2.8.1, you would have to change your version to “2.8.2-dev”.
If you are feeling really experimental, you can get on the 2.9 development path by setting the version to “2.9-dev”. Unlike the 2.8 development path which contains only fixes to high-impact bugs, the 2.9 path is where new feature development is happening. You probably want the 2.9 path only if you are a developer.
Update: Peter wrote a plugin that makes this easier.