I'll take that as a "no". What i mean is to actually run exactly one cluster (no replicated copy) on more than one server. Of course, if that were possible, why would people bother with replication..
I guess it is irrational to suggest that it would be possible, since each server would at least need to have it's own copy of the DBMS software etc, or it would cease to be a separate server.
Maybe "Data Partitioning", as in the documentation link Shane sent, possibly combined with Slony for the other data per server, would be an option for me.
Is there an implementation for this in PostgreSQL? It would have to be something like pgPool (middleware), because: How would the query know on which server to put it's data? And i guess i would need some Kerberos-like implementation for my authentication and authorization...
There used to be parallel server type solution for postgresql called ExtenDB www.extendb.com , but i think its now acquired by a commercial PostgresSQL vendor, but i have seen others similar to that.