Follow-up of a discussion on dev-docs@ :

Don't show PSEUDO_THREAD_ID in SHOW VARIABLES because:
- we don't want people to discover this variable as it could never do good to
set it (it was designed for use by mysqlbinlog only, so that a thread can have
several temp tables of the same name at the same time)
- if we show it in SHOW VARIABLES, Mysql Administrator will display it and
this will force us to put a description, so all MySQL Administrator user
will be aware of this variable, some may have the idea to set it with a SET
command, and then it will cause bad things.
The variable is still settable, and still visible with SELECT @@.

sql/set_var.cc

@@ -695,7 +695,6 @@ struct show_var_st init_vars[]= {
   {"port",                    (char*) &mysqld_port,                  SHOW_INT},
   {"protocol_version",        (char*) &protocol_version,            SHOW_INT},
   {sys_preload_buff_size.name, (char*) &sys_preload_buff_size,      SHOW_SYS},
-  {sys_pseudo_thread_id.name, (char*) &sys_pseudo_thread_id,        SHOW_SYS},
   {sys_query_alloc_block_size.name, (char*) &sys_query_alloc_block_size,
    SHOW_SYS},
 #ifdef HAVE_QUERY_CACHE