After submitting this bug, I discovered that 
"imp/config/prefs.php.dist" had been updated as part of 
<URL:http://bugs.horde.org/ticket/?id=%238936>. When I merged the 
changes into my working "imp/config/prefs.php", message display 
worked, so this isn't really a bug.

In IMP or DIMP, the Inbox displays "There are no messages in this 
mailbox." though there are many messages. Some other folders display 
messages, and some display none, like the Inbox. Whenever a folder is 
incorrectly displayed as empty, I see the following in my web server's 
error log:
-------------------------------------------------------------------------------------------
[Tue Mar 30 03:15:26 2010] [error] [client xx.xx.xx.xx] PHP Warning:   
array_map() [<a href='function.array-map'>function.array-map</a>]: 
Argument #2 should be an array in /var/lib/horde/imp/lib/Mailbox.php 
on line 136, referer: 
https://example.com/horde/services/portal/sidebar.php
[Tue Mar 30 03:15:26 2010] [error] [client xx.xx.xx.xx] PHP Stack 
trace:, referer: https://example.com/horde/services/portal/sidebar.php
[Tue Mar 30 03:15:26 2010] [error] [client xx.xx.xx.xx] PHP   1. 
{main}() /var/lib/horde/imp/mailbox.php:0, referer: 
https://example.com/horde/services/portal/sidebar.php
[Tue Mar 30 03:15:26 2010] [error] [client xx.xx.xx.xx] PHP   2. 
IMP_Mailbox->shutdown() /var/lib/horde/imp/lib/Mailbox.php:0, referer: 
https://example.com/horde/services/portal/sidebar.php
[Tue Mar 30 03:15:26 2010] [error] [client xx.xx.xx.xx] PHP   3. 
array_map() /var/lib/horde/imp/lib/Mailbox.php:136, referer: 
https://example.com/horde/services/portal/sidebar.php
-------------------------------------------------------------------------------------------

Using git-bisect, I have tracked the problem down to 
c5d8fa67cd3937f6e596a7e722c7612074332094 which involves a number of 
framework IMAP client files, so it seems that either the changes 
unintentionally broke interfaces IMP uses or IMP needs to be updated 
to use new interfaces.