I suppose you could overwrite the files that changed in in 2.9.2 with the original files from the 2.9 build - I doubt any functionality was added in 2.9.2. Before trying though, I strongly suggest backing everything up!
you may be right that there may not have had any functionality added, but usually there are security fixes and the such. however, Yes i would either reinstall the whole installation OR wait for WP 3.0. It is sure to have more functionality and more security features to protect against hackers and the such. But it is VERY UNWISE to revert back to an older version of WP. Very unsafe and insecure.
i donot advice you to do it , but if you need to do it first download 2.9 version http://wordpress.org/wordpress-2.9.zip extract then repalce the new files with the new files , do not forget to de-activate all the plugins , and back up files and data base regards
Yep just overwrite the existing files with whatever version you want just as you would do a manual upgrade. You can go back as far as you want but go back too far and you may experience plugin and or theme compatibility issues. But Do Not overwrite the themes and the plugin folders that are in the wp-content folder, that will wipe out all your existing plugins and theme(s). But as stated above Backup. Backup at least the wp-content folder, the wp-config.php file (in the main directory) and the database. I have had occasions to revert back to older versions for plugin functionality that hasn't caught up to the newer version of WP.