Bitmap indexes were planned for version 8.3, but were not ready. Now they are expected in version 8.4.
Looks like they've been postponed until 8.5? It's still on the ToDo List
The create index command will probably be of the form:
CREATE INDEX tab1idx ON tab1 USING bitmap (col1,col2);
There are multiple solutions available to run a cluster of PostgreSQL servers. The two solutions that are cited most often are GridSQL and pgpool-II. GridSQL seems to be better suited for Reporting / data mart / OLAP workloads, whereas pgpool-II seems to be better suited for transactional workloads.
It is even reported that running multiple logical nodes of GridSQL on one (beefy) server box can improve performance, see GridSQL and Muticore architectures.