Ups!, yes, you are right. I'm very sorry, seems that I've had some 
problems with the update to the last git sources

This should already been fixed in Git by this commit: 
http://git.horde.org/horde-git/-/commit/bccd40d56f9e21beebeee045a50fe6571cd68dd3?rt=horde-git&ws=1

Well after debugging the value of $driver in this line it gets a null 
value when printing the login page or after logout, once logged in it 
gets the value 'Horde_Prefs_Storage_Sql'

In my configuration I'm using IMP as the authentication method

If I add a conditional if ($driver) before the code all seems to work 
fine, I've revised previous versions of this code and the call to "new 
$driver..." was previously executed inside a conditional assignment:
             $drivers = $driver
                 ? array(
                       $config_driver,
                       new $driver($opts['user'], $params),
                       $hooks_driver
                   )
             ....

Hope this helps

I've regenerated the configuration files but I continue having the 
same problem
What of the various defined drivers in the horde/config/conf.php is this?
$conf['auth']['driver'] = 'application';
$conf['prefs']['params']['driverconfig'] = 'horde';
$conf['prefs']['driver'] = 'Sql';
$conf['alarms']['params']['driverconfig'] = 'horde';
$conf['alarms']['driver'] = 'Sql';
$conf['datatree']['driver'] = 'null';
$conf['group']['driverconfig'] = 'horde';
$conf['group']['driver'] = 'Sql';
$conf['perms']['driverconfig'] = 'horde';
$conf['perms']['driver'] = 'Sql';
$conf['share']['driver'] = 'Sqlng';
$conf['cache']['driver'] = 'Apc';
$conf['lock']['params']['driverconfig'] = 'horde';
$conf['lock']['driver'] = 'Sql';
$conf['token']['params']['driverconfig'] = 'horde';
$conf['token']['driver'] = 'Sql';
$conf['vfs']['params']['driverconfig'] = 'horde';
$conf['spell']['driver'] = '';
$conf['image']['driver'] = false;
$conf['exif']['driver'] = 'Bundled';
$conf['accounts']['driver'] = 'null';

How is your driver name null?  That is not a valid driver name.   
"Null" (a string) is proper.  Sounds like your horde/config/conf.php 
is broken.

After updated to the last GIT sources I've receive an error when 
connecting to horde and the login screen is not shown

The error in the apache log says:

[Fri May 06 12:57:04 2011] [error] [client A.B.C.D] PHP Fatal error:   
Class name must be a valid object or a string in 
/usr/share/php5/PEAR/Horde/Core/Factory/Prefs.php on line 118

In my case de value of "$driver" is null, I don't know if this is normal