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Yes, PostgreSQL does work with WebObjects. The best place to start is the Project WOnder PostgresqlPlugin Page.
If you're on a Mac, Apple has an article on installing PostgreSQL on Mac OS X that is pretty straightforward.
Mark Liyanage builds an installer for postgresql. Have a look at entropy
Another way to get postgresql installed is to use one of the installers that maintain a complete ports tree. There is Fink and Macports. Bot can be used via a graphical installer, in fink it is included. For macports you can use porticus.
If you're interested in a nice graphical interface to managing your PostgreSQL installation, you may want to checkout out Navicat PostgreSQL from Navicat.