I love using the automatic upgrade path in WordPress but can you use it to upgrade to a beta version. Yes you can. Using an ftp programme navigate to wp-includes and edit version.php. Find this line:
$wp_version = '2.8';

It is towards the top and defines what version you are using. Simply change it to whatever beta version you require.
$wp_version = '2.8.1-dev';

See Nightly Builds for more information.