Just had a rather nasty experience trying to upgrade to WP 2.8 using
svn switch to move to the 2.8 tag – it resulted in the whole thing going pear shaped during the upgrade process and complaining about:
Fatal error: Class ‘WP_Widget’ not found in $wordpress/wp-includes/default-widgets.php on line 15
Turns out that the files in SVN don’t match what’s in the 2.8 tarball, and the 2.8 tag in SVN doesn’t work. The fix was to rsync the files from the tarball over SVN. Very very weird (and the first time I’ve had such problems!).