Which installation parts are backward compatible?

I've been examining multi-major-version binary packaging again, and I was 
wondering whether we have a good overview over which pieces of the 
installation are backward compatible (that is, they can be shared between all 
major versions) and which are not.  For example, psql 8.4 can now presumably 
be shared between all major versions, so previous schemes to have several 
psqls installed can be simpified.

Here is a start:

Item        Compatible across major versions?    (i.e. the 8.4 version works with 7.4+ server)

clusterdb    yes?
createdb    yes?
createlang    yes?
createuser    yes?
dropdb        yes?
droplang    yes?
dropuser    yes?
ecpg        no?
initdb        NO
pg_config    (should be kept around for each major version)
pg_controldata    NO
pg_ctl        ?
pg_dump        read yes, write NO
pg_dumpall    read yes, write NO
pg_resetxlog    NO
pg_restore    yes?
pgtcl_*mod    YES
postgres    NO
psql        YES
reindexdb    yes?
vacuumdb    yes?

If no such list exists yet, perhaps we can complete the above one, document 
it, and pass it on to the packagers.