Changes have been made in Git (master):

commit fc630cde3cc879d1006a1c7f4b3f147096037a86
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Wed Feb 20 12:49:51 2013 -0700

     Allow GarbageCollection task to be run from the command line (Bug #12061)

  .../LoginTasks/SystemTask/GarbageCollection.php    |   21 ++++++++++++-------
  1 files changed, 13 insertions(+), 8 deletions(-)

http://git.horde.org/horde-git/-/commit/fc630cde3cc879d1006a1c7f4b3f147096037a86

Fixed an exception from being thrown, although running that particular 
task from the command line won't do all tasks.  A password for that 
user is required for several of the tasks and that is not something an 
admin would know for anyone besides themselves on a stock IMAP/POP 
setup.

I am trying to manually run GarbageCollection on IMP. I saw that you 
already have a script to run tasks and tried this:

horde/bin/horde-run-task -a imp -t GarbageCollection -u 
rui.carneiro@portugalmail.net

And got the following error:

Fatal error: In imp/lib/Imap.php on line 344

  1. IMP_LoginTasks_SystemTask_GarbageCollection->execute() 
horde/bin/horde-run-task:90
  2. Horde_Injector->getInstance() 
imp/lib/LoginTasks/SystemTask/GarbageCollection.php:38
  3. Horde_Injector->createInstance() 
framework/Injector/lib/Horde/Injector.php:247
  4. Horde_Injector_Binder_Factory->create() 
framework/Injector/lib/Horde/Injector.php:213
  5. IMP_Factory_Imaptree->create() 
framework/Injector/lib/Horde/Injector/Binder/Factory.php:111
  6. IMP_Imap_Tree->__construct() imp/lib/Factory/Imaptree.php:57
  7. IMP_Imap_Tree->init() imp/lib/Imap/Tree.php:181
  8. IMP_Imap_Tree->_getList() imp/lib/Imap/Tree.php:272
  9. IMP_Imap->listMailboxes() imp/lib/Imap/Tree.php:310
10. IMP_Imap->__call() imp/lib/Imap/Tree.php:310

This is a bug or it is intended since I do not provide a password to 
login into the user account?