Thanks to Richard Moon, Chris McDonough and Dan Wickstrom for info and comments re ODBC drivers.
Following from the Postgresql docs it appears that there are available open source client ODBC drivers for FreeBSD and Linux as well as for Windows. See:
ftp://ftp.postgresql.org/pub/odbc/psqlodbc_faq.html
http://users.ids.net/~bjepson/freeODBC/
http://www.iodbc.org/index.htm
The main open source one appears to be maintained by the same openlinks people that distribute the commercial ODBC drivers Chris mentioned, so perhaps same code base?
There was also a mention of ApplixWare, which I don't know anything about.
Based on the other comments, I gather that Zope just needs to be able to use an OS specific ODBC client driver and not a Zope specific one, so it seems there is no actual barrier to using Zope on any platform with any RDBM on any platform, apart from the usual non-commercial support hassles (and corresponding benefits).
If that's not a good bet, please correct me.
PS This of course has nothing to do with data abstraction etc. Will try to follow up on that and other issues after done some more reading. Just wanted to provide those ODBC links immediately.
