varchar as primary key

I'm investigating the usage of a UUID primary key generator using
Hibernate and Postgres.  The reason for using a UUID is that we will
have an application hosted at different sites in different
databases.  We will need to aggregate the data back into a single
database from time to time and we want to avoid PK collisions.
   Is there a significant performance difference between using int
primary keys and string primary keys in Postgres?
   Thanks,
    -M@