If you have updated Boost to version 1.50.0 via MacPorts, you may have encountered something like the following:

$ port installed | grep mongodb
  mongodb @2.0.6_0 (active)
$ port installed | grep boost
  boost @1.50.0_0 (active)
$ mongo
dyld: Symbol not found: __ZNK5boost15program_options16validation_error4whatEv
  Referenced from: /opt/local/bin/mongo
  Expected in: /opt/local/lib/libboost_program_options-mt.dylib
 in /opt/local/bin/mongo

There are numerous reports on the MacPorts Trac with various ports that are affected. Here is the solution that worked for me:

$ cd /tmp
$ svn co  -r 93341 'http://svn.macports.org/repository/macports/trunk/dports/devel/boost/'
A    boost/files
A    boost/files/patch-boost-foreach.diff
A    boost/files/patch-thread_visibility.diff
A    boost/files/patch-libs-mpi-build-Jamfile.v2.diff
A    boost/files/patch-tools-build-v2-tools-python-2.jam.diff
A    boost/files/patch-tools-build-v2-tools-python.jam.diff
A    boost/files/patch-bootstrap.sh.diff
A    boost/files/patch-tools_build_v2_engine_src_build.jam.diff
A    boost/files/patch-libs-python-src-converter-builtin_converters.cpp
A    boost/files/patch-tools_build_v2_engine_src_build.sh.diff
A    boost/Portfile
Checked out revision 93341.

$ cd boost
$ sudo port install
--->  Computing dependencies for boost
--->  Fetching archive for boost
--->  Attempting to fetch boost-1.49.0_0.darwin_10.x86_64.tbz2 from http://packages.macports.org/boost
--->  Attempting to fetch boost-1.49.0_0.darwin_10.x86_64.tbz2.rmd160 from http://packages.macports.org/boost
--->  Installing boost @1.49.0_0
--->  Deactivating boost @1.50.0_0
--->  Cleaning boost
--->  Activating boost @1.49.0_0
--->  Cleaning boost

$ sudo port activate boost @1.49.0_0
--->  Computing dependencies for boost
--->  Cleaning boost