Camera in volume is now natively supported as of 2.73:
Cycles: Add support for cameras inside volume
Basically the title says it all, volume stack initialization now is
aware that camera might be inside of the volume. This gives quite
noticeable render time regressions in cases camera is in the volume
(didn't measure them yet) because this requires quite a few of
ray-casting per camera ray in order to check which objects we're
inside. Not quite sure if this might be optimized.
But the good thing is that we can do quite a good job on detecting
whether camera is outside of any of the volumes and in this case there
should be no time penalty at all (apart from some extra checks during
the sync state).
For now we're only doing rather simple AABB checks between the
viewplane and volume objects. This could give some false-positives,
but this should be good starting point.
Need to mention panoramic cameras here, for them it's only check for
whether there are volumes in the scene, which would lead to speed
regressions even if the camera is outside of the volumes. Would need
to figure out proper check for such cameras.
There are still quite a few of TODOs in the code, but the patch is
good enough to start playing around with it checking whether there are
some obvious mistakes somewhere.
Currently the feature is only available in the Experimental feature
sey, need to solve some of the TODOs and look into making things
faster before considering the feature is ready for the official
feature set. This would still likely happen in current release cycle.
As user sambler pointed out, fix is on the way.