To add a couple of points about Gemstone:

Gemstone can execute code on the server (in fact, that's what it's
designed to do).  So if you do need to execute some block across a
large database, there's no need to fault it all into an image.  It
sounds as if the other object db's that have been mentioned don't
operate that way.

There is a non-commercial verison of Gemstone and, I think, a
front-end for squeak--or at least that was my impression a year or two
ago.

Gemstone is the object db I'm most familiar with, but I've been trying
to stay with open-source solutions.