The coming WordPress Version 2.7 has the popular required core update included. You can already try out the functions.
But sometimes you don’t want or can’t work online. Here is a little workaround how to develop with WordPress 2.7 Beta offline.
Since you cannot enter the backend I suggest to comment out a hook in /wp-includes/update.php .
add_action( 'init', 'wp_version_check' );
...
add_action( 'load-plugins.php', 'wp_update_plugins' );
add_action( 'admin_init', '_maybe_update_plugins' );
After that you can enter the backend without problems. Everybody who works daily with it and gets the newest update, which makes sense if you use SVN, doesn’t want to comment out this entry every time.
Alternative you can use a Plugin. You can copy and use an easy and fast variant below. Just paste it in your Plugin folder and activate. Then you can irreversible the change in update.php and there won’t be a version check anymore.
The currect version can xou found on github – https://github.com/bueltge/WP-Offline