protocol compatibility between 7.2 and 7.4

I know this is an attempt to save myself reading the mailing list, but still 
the issue remains:

the psql from version 7.4 does not talk to a 7.2.4 database.

The CHANGELOG indicates, that both server and libraries keep compatibility 
with versions after 6.3 - still there is no switch in psql to specify usage of 
the pre-7.3 protocol and the 7.2 server and 7.4 psql apparently do not 
negotiate.

Daniel