Adding foreign key constraints without integrity check?

Is there a way to add a foreign key constraint without having to wait for it
to check the consistency of all existing records?  If a database is being
reloaded (pg_dumpall then load), it really shouldn't be necessary to check
the referential integrity - or at least I should be able to stipulate that I
am accepting that risk.

My database reload is currently taking about 6 hours to load the data, 42
hours to reindex, and about another 40 hours or so to check the foreign key
constraints (about 1.2 billion rows).  That's a very long time to be out of
commission.  I'd really like to eliminate that second 40 hours so I can get
it down to a normal weekend.

Wes