Changes have been made in Git (develop):

commit 74cc39021178c384a740fe56d7a131231908e29b
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Sep 5 18:20:05 2012 -0400

     This value is technically an integer.

     Possible fix for Bug: 11408

  nag/lib/Nag.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/74cc39021178c384a740fe56d7a131231908e29b

Changes have been made in Git (develop):

commit 007f9640da615eb0590a50ff4779dd7098c0ac09
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Sep 5 17:47:51 2012 -0400

     Ensure _pointer is initialized to zero.

     Fixes a chicken-egg issue when calling reset with children present.
     Possible fix for Bug: 11408

  nag/lib/Task.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/007f9640da615eb0590a50ff4779dd7098c0ac09

Yes, that seems to work.  At a minimum, the default tasklist now 
appears in the select element on the prefs page.  Thanks for helping 
track this down.

Still can't reproduce, but does my recent change fix for you?

Changes have been made in Git (master):

commit 74cc39021178c384a740fe56d7a131231908e29b
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Sep 5 18:20:05 2012 -0400

     This value is technically an integer.

     Possible fix for Bug: 11408

  nag/lib/Nag.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/74cc39021178c384a740fe56d7a131231908e29b

Same thing in Nag_View_List#render().  This causes my tasklist to 
appear again in the tabs:

//            if ($list->get('issmart')) {
                  $tabs->addTab(
                      $list->get('name'),
                      $listurl->add(array('actionID' => 'smart', 
'list' => $list->getName())),
                      array('img' => 'search.png'));
//            }

I'm using Sqlng.

Determined that this is the issue.  Commenting this line out in 
Nag::listTasklists() fixes things:

         if (!$smart) {
//            $att['issmart'] = '0';
         }

This sounds more like a Shares issue.  Which share backend are you 
using? sql or sqlng?

Still can't reproduce this.

Have you updated to the most recent migration? There were a number of 
new migrations recently.

Going to preferences -> Default Task List, the drop-down select box is 
completely empty.  Not sure if this is the issue.

I can verify that all migrations have been run, conf.php is 
up-to-date, and I am using the base configuration files.

None of them show anything.

Going back about 300 commits in history (git checkout HEAD~300). I 
notice that the "default" tasklist appears in the sidebar again 
("Michael M. Slusarz's Tasklist") and my tasks are displayed.  If I 
then revert to master, this tasklist remains selected in my sidebar 
and the tasklist continues to display.  Once I deselect that tasklist 
for display, or click on View tasklists, this default tasklist 
disappears and again I can see no tasks.

Changes have been made in Git (master):

commit 007f9640da615eb0590a50ff4779dd7098c0ac09
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Sep 5 17:47:51 2012 -0400

     Ensure _pointer is initialized to zero.

     Fixes a chicken-egg issue when calling reset with children present.
     Possible fix for Bug: 11408

  nag/lib/Task.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/007f9640da615eb0590a50ff4779dd7098c0ac09

Also, which tab are you selecting to view (All, Incomplete etc...)?

I can't reproduce this. From your second comment it sounds like you 
might have some stale or corrupt session data for the Tag Browsing 
functionality. Have you tried starting with a fresh session?

List tasks page lists 0 tasks and this text:

There are no tasks matching the current criteria.

No tasks match

I can confirm there are tasks: I can link directly through tasks via 
the portal block, which does show tasks, and via the link in the 
e-mail notifications.  But the task page in the nag application itself 
is always empty.  No error messages in the Horde log (log level DEBUG).