Changes have been made in CVS for this ticket:

http://cvs.horde.org/diff.php/framework/Crypt/Crypt/smime.php?r1=1.49.2.18&r2=1.49.2.19&ty=u

I didn't write any of this code (sorry) - but mucking through the 
attached data, looks like this is the difference between the two 
pertinent data objects is as follows:

Bad data:
[9] => Array
                                 (
                                     [0] => bit string
                                     [1] => +
                                 )

Good data:
                             [9] => Array
                                 (
                                     [0] => sequence
                                     [1] => Array
                                         (
                                             [0] => Array
                                                 (
                                                     [0] => oid
                                                     [1] => 
id-ce-basicConstraints
                                                 )

                                             [1] => Array
                                                 (
                                                     [0] => boolean
                                                     [1] => 1
                                                 )

                                             [2] => Array
                                                 (
                                                     [0] => octet string
                                                     [1] => 0�
                                                 )

                                         )

                                 )

The code has something to do with determining certificate extensions I 
guess, so I just ignored data formatted as the former to fix the 
error.  Others who know more about S/MIME can feel free to fix 
properly, as necessary.

Changes have been made in CVS for this ticket:

http://cvs.horde.org/diff.php/framework/Crypt/Crypt/smime.php?r1=1.71&r2=1.72&ty=u

Michael, is this something you could take a look at?

versions and the like are:

Horde Version
Horde: 3.2-RC3

Horde Applications
Dimp: H3 (1.0-RC3)
Horde: 3.2-RC3
Imp: H3 (4.2-RC3)
Ingo: H3 (1.2-cvs))
Kronolith: H3 (2.2-RC2)
Mimp: H3 (1.0-cvs)
Nag: H3 (2.2-cvs)
Turba: H3 (2.2-cvs)

PHP Version
PHP Version: 5.2.0-8+etch10

in particular, smime.php:
* $Horde: framework/Crypt/Crypt/smime.php,v 1.49.2.17 2008/01/02 
11:30:26 jan Exp $


the exact error message is simply this (when trying to *view* a 
message in IMP):
Apr 21 13:44:27 ### apache2: PHP Fatal error:  Cannot use string 
offset as an array in /###/horde.org/lib/Horde/Crypt/smime.php on line 
827


however, it seems that the error only occurs with one of my 
certificates, i.e. (and that's probably what makes it hard to 
reproduce) messages signed with one of my certificates just display 
fine and messages signed with another one fail - sorry, hadn't 
realized this before...

so: i attached the two certificates (der format) and two message 
sources (names should be rather obvious) where one displays fine and 
the other fails with the mentioned error message.

Can you please provide a full source message that we can use to 
reproduce the problem?

(OOPS - sorry) a possible solution:

*** smime.php.org
--- smime.php
***************
*** 825,830 ****
--- 825,831 ----
           }

           $i = 9;
+                   $cert_data[1][0][1] = array();
           while (isset($cert_data[1][0][1][$i])) {
               $oid = $cert_data[1][0][1][$i][1][0][1];
               $cert_details['certificate']['extensions'][$oid] = 
$cert_data[1][0][1][$i][1][1];

when trying to view a s/mime signed message in imp (at least this 
holds for my own signed messages - however, signed with a different 
application) i get the mentioned php fatal error:

a possible solution:

         $i = 9;
         $cert_data[1][0][1] = array();
         while (isset($cert_data[1][0][1][$i])) {
             $oid = $cert_data[1][0][1][$i][1][0][1];
             $cert_details['certificate']['extensions'][$oid] = 
$cert_data[1][0][1][$i][1][1];
             $i++;
         }