Actually, you do not need to set BOOST_ROOT. CMake can find Boost in its standard location. It worked for me on Ubuntu 12.04 LTS, with CMake version 2.8.7 and Boost 1.46.
Theoretically, BOOST_ROOT should be set to /usr if for some reason the above won't work.
Should you have a problem, then add -DBoost_DETAILED_FAILURE_MSG=TRUE to the cmake invocation (note the spelling!). This will print some moderately helpful error messages.