I just ran across the user table configuration setting in the WordPress codex, and thought that it was pretty cool. Essentially, you can have one or more blogs that all use the same user tables and user meta data. If you want the blogs to use the same user base, but want different user meta for each additional site, you only need to modify the CUSTOM_USER_TABLE to point to the original site, and leave the meta table alone, which will then create the meta table inside the new site.
define( 'CUSTOM_USER_TABLE', $table_prefix.'my_users' ); define( 'CUSTOM_USER_META_TABLE', $table_prefix.'my_usermeta' );