A database created under python 2.5 cannot be opened under python 2.6. It gives the error message "DB_RUNRECOVERY: Fatal error, run database recovery -- process-private: unable to find environment ", and a database created under python 2.6 cannot be opened under python 2.5 (see http://trac.macports.org/ticket/24310). (This in in Mac OS X: In Windows XP SP3, Python 2.6 can read a Python 2.5 bsddb data base.
but not the other way around. If you try, you will end up with a corrupt data base.)

python 2.6 bsddb is very much slower than python 2.5. Specifically, in Gramps, import of a 500 person xml file takes 12 sec with python25 and 9 mins 30 secs with python26. The slowness has been observed in Mac OS X (See http://trac.macports.org/ticket/23768) and in Windows (see http://www.gramps-project.org/bugs/view.php?id=3750).

I am not sure, but I think that both systems are using the same underlying database module db46, and that the difference may be in the different interface modules: "_bsddb.so" (on Mac OS X)