aptitute "Unable to correct dependencies" -- how to fix?

[Adam Funk <a24061@yahoo.com>]

For a while now I've been getting the problem below.

My /etc/apt/apt.conf contains only two lines.

APT::Default-Release "testing";
APT::Clean-Installed "off";

My /etc/apt/preferences has one hand-coded pin and a bunch produced by
apt-listbugs.  I tried removing the hand-coded pin and still got the same
result.

# aptitude dist-upgrade
Reading Package Lists... Done
Building Dependency Tree       
Reading extended state information       
Initializing package states... Done
Reading task descriptions... Done  
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused
by held packages.
E: Unable to correct dependencies, some packages cannot be installed

# aptitude -f -D dist-upgrade
Reading Package Lists... Done
Building Dependency Tree       
Reading extended state information       
Initializing package states... Done
Reading task descriptions... Done  
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused
by held packages.
E: Unable to correct dependencies, some packages cannot be installed

I would appreciate suggestions on what to try next in order to track down
the problem.

Thanks,
Adam