[ADMIN] Where are security permissions stored ?

[Thorne, Francis]

In postgres 8.3.6 could you please tell me where the security permission 
details for users are stored (i.e. who has select on a particular table), for 
example in the master database in M-SQL, am I right in thinking these details 
are stored in pg_catalog and if so I'm guessing this should be backed up

Thanks
FT