=dev-libs/boost-1.51* should set SLOT="0/1.51" so that := dependencies in packages depending on dev-libs/boost provide usual benefits.
Created attachment 328380 [details, diff] boost.patch Patch backported from Progress Overlay.
Sonames of Boost libraries contain micro version (e.g. libboost_system.so.1.52.0 instead of libboost_system.so.1.52), so maybe SLOT should be rather "0/1.52.0" instead of "0/1.52".