Postgres clustering?

I am interested in finding out about Enterprise scale Postgres
installations and clustering, especially on Linux.

Essentially I would like to know the possibility that Postgres can store
the database data in a central location (ex: on a SAN fiber array) and
have a cluster of machines sharing processor/RAM/IO bandwidth to do the
application processing. Or perhaps there is another solution similar to
what www.emicnetworks.com have developed for MySQL where the 'cluster'
seems to be really a set of individual MySQL servers that syncronize
their data at a very high speed?

I have found from my investigations that the usual Linux cluster tools,
like Beowulf/Mosix/openMosix/Alinka/Qlusters dont support Postgres and
similar apps because of the shared memory or other issues. The
complexity of RDBMS transactions seems to have limited parallel
processing in this field to the big (read: EXPENSIVE) guys like Oracle
and IBM.

Does anyone know of a technique to centralize the Postgres data storage
and farm out the processor/RAM/IO needs to a scalable resource? It seems
like it should be able to be done, but at the moment its looking to me
like the choice looks like this:

- Buy bigger and bigger hardware to run Postgres.
- Fork out large sums of money and port applications to Oracle or DB2

Anyone have any success with OpenSource tools for this or similar
issues?

Thanks.