Sharing user tables in WP2.0

It should be possible for several installs of WordPress to use the same user tables. I kinda got it to work thanks to this topic, however, in WP2.1 this no longer seems to work. Whenever I log in into one install using a user created with another install, I get the message “You do not have sufficient permissions to access this page”. The user has full admin rights and I’ve followed all the instructions as given by the user Yoshi.