shared temp tables

I love the fact that temp tables do not exist in every PostgreSQL session,
don't get me wrong. 

The issue is this: most "web environments" have the idea of a session. A
session management scheme based on PostgreSQL exposes PostgreSQL's worst
behavior. Small amount of records, high update/delete rate for each record. So
much so, that it probably isn't realistic to replace something like Oracle with
PostgreSQL in this environment.

Do "temp tables" suffer the same delete/update behavior of marking the row as
deleted and adding another row? Thus requiring vacuum periodically. 

If not, should/could there be a way to create a temp table that is globally
visible?