This is a recent post from the web/db bboard at aD.
It is nice to hear that aD will be supporting PostgreSQL in the
future...
"I would not say precisely that database independence is a main
focus for 4.6, but the object-relational mapping provided by the
persistence system is. 4.6 will not be database independent in
the sense that it will only work on Oracle, and we do not intend to
port 4.6 to any other databases. However, there will be an
upgrade path to 5.0 and we do intend to provide Postgres
support for 5.0 within a couple months of the initial release."
http://www.arsdigita.com/bboard/q-and-a-fetch-msg?msg%5fid=000eQo&topic%5fid=21&topic=web%2fdb