The current issue of Linux Journal has an article by Joshua Bentham on
"Easy Database Development Using Rekall". I quote:
The database back end was a no-brainer; I used PostgreSQL.
Having access to PostgreSQL version 8 under both Linux and
Windows is not only a testament to the power of great free
software, but it also provides an extremely fertile ground upon
which to grow cross-platform applications that are robust and
scalable.
(Also, Rueven Lerner talks about using PostgreSQL with iCalender
data.)