|Thank you for your answer.
# apt-get -o Debug::pkgProblemResolver=yes dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Starting
Investigating (0) libboost1.46-dev [ amd64 ] < none -> 1.46.1-6 > ( libdevel )
Broken libboost1.46-dev:amd64 Conflicts on libboost1.42-dev [ amd64 ] < 1.42.0-4+b1 > ( libdevel )
Considering libboost1.42-dev:amd64 0 as a solution to libboost1.46-dev:amd64 0
Holding Back libboost1.46-dev:amd64 rather than change libboost1.42-dev:amd64
Investigating (0) libboost-dev [ amd64 ] < 220.127.116.11 -> 18.104.22.168 > ( libdevel )
Broken libboost-dev:amd64 Depends on libboost1.46-dev [ amd64 ] < none -> 1.46.1-6 > ( libdevel )
Considering libboost1.46-dev:amd64 0 as a solution to libboost-dev:amd64 0
Holding Back libboost-dev:amd64 rather than change libboost1.46-dev:amd64
Try to Re-Instate (1) libboost-dev:amd64
The following packages have been kept back:
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
My apt wizardry is lacking, I've no idea how serious this is (everything works fine) or how to investigate further.
If I've screwed up somewhere it might have to do with enabling a multimedia repo a year ago or so. I don't develop in C++ but -dev packages may have been installed to compile 3rd party source code.