Fixed.

http://lists.horde.org/archives/commits/2010-December/005841.html

this doesn't work for me.

framework/Core/lib/Horde/Registry.php:1357
php seams to be running code up to $prefs->retrieve($app), but never returns.

here is the output of print_r($prefs) just before calling 
$prefs->retrieve($app) (login, wait 2 seconds and kill httpd) :
HORDE [imp] Horde_Prefs Object
(
     [_cache:protected] => Horde_Core_Prefs_Cache_Session Object
         (
             [_params:protected] => Array
                 (
                     [user] => rsalmon
                 )
         )
...
     [_storage:protected] => Array
         (
             [0] => Horde_Core_Prefs_Storage_Configuration Object
                 (
                     [hooks] => Array
                         (
                             [imp] => Array
                                 (
                                     [0] => search_sources
                                     ...
                                     [140] => search_sources
                                 )
                         )
                     [_params:protected] => Array
                         (
                             [user] => rsalmon
                         )
                 )
             [1] => Horde_Prefs_Storage_Sql Object
             ...
             [2] => Horde_Core_Prefs_Storage_Hooks Object
                 (
                     [_params:protected] => Array
                         (
                             [conf_ob] => 
Horde_Core_Prefs_Storage_Configuration Object
                                 (
                                     [hooks] => Array
                                         (
                                             [imp] => Array
                                                 (
                                                     [0] => search_sources
                                                     ...
                                                     [140] => search_sources
                                                 )

                                         )

                                     [_params:protected] => Array
                                         (
                                             [user] => rsalmon
                                         )

                                 )

                             [user] => rsalmon
                         )

That hook does not work because it is causing the infinite loop.  You 
can't get the value of the preference with a getValue() call when the 
preference hasn't been set yet.  Duh, Michael.

That being said, there does need to be a way to indicate from the hook 
that the original preference value should be used.  I originally 
committed something to allow returning null to indicate the original 
value should be used.  But it makes more sense to pass the original 
value in, since it may be useful to alter the original value.

file attached

What is the contents of your prefs hook?

with the following setting and hook.php configure using the provided exemple
$_prefs['search_sources'] = array(
     'value' => '',
        'locked' => true,
        'hook' => true
);

When I try to login I get (indefinitely) in /tmp/horde.log :

2010-12-06T17:20:54+01:00 DEBUG: HORDE [imp] Hook prefs_init in 
application imp called. [pid 28875 on line 1688 of 
"/var/www/html/horde/libs/Horde.php"]
2010-12-06T17:20:54+01:00 DEBUG: HORDE [turba] SQL  (0,0004s)
         SELECT COUNT(*) FROM horde_perms WHERE perm_name =
           'turba:sources:localldap' [pid 28875 on line 754 of 
"/var/www/html/horde/libs/Horde/Db/Adapter/Base.php"]
2010-12-06T17:20:54+01:00 DEBUG: HORDE [imp] SQL  (0,0007s)
         SELECT pref_scope, pref_name, pref_value FROM horde_prefs WHERE
           pref_uid = 'rsalmon' AND pref_scope = 'imp' [pid 28875 on 
line 754 of "/var/www/html/horde/libs/Horde/Db/Adapter/Base.php"]
2010-12-06T17:20:54+01:00 DEBUG: HORDE [imp] Hook prefs_init in 
application imp called. [pid 28875 on line 1688 of 
"/var/www/html/horde/libs/Horde.php"]
2010-12-06T17:20:54+01:00 DEBUG: HORDE [turba] SQL  (0,0004s)
         SELECT COUNT(*) FROM horde_perms WHERE perm_name =
           'turba:sources:localldap' [pid 28875 on line 754 of 
"/var/www/html/horde/libs/Horde/Db/Adapter/Base.php"]
2010-12-06T17:20:54+01:00 DEBUG: HORDE [imp] SQL  (0,0008s)
...


From what I could track down :
Registry.php, function popApp(), php seams to be running code up to 
line 1241 :
  $this->_onAppSwitch($app).






Nothing to do with this ticket, but there is a typo in 
framework/Ldap/lib/Horde/Ldap/Entry.php
--- Entry.php.old        2010-12-06 17:30:46.000000000 +0100
+++ Entry.php        2010-12-06 17:30:51.000000000 +0100
@@ -289 +289 @@
-            for ($attr = @ldap_first_attribute($this->_link, 
$this->_entry, $ber);
+            for ($attr = @ldap_first_attribute($this->_link, $this->_entry);
@@ -291 +291 @@
-                 $attr = @ldap_next_attribute($this->_link, 
$this->_entry, $ber)) {
+                 $attr = @ldap_next_attribute($this->_link, $this->_entry)) {