disallowing multiple NULLs in a unique constraint

SQL Server only allow one NULL in a unique constraint column (it's the 
unique index that does that, so the unique constraint behaves like that 
too). The question is, what is the best way to simulate that behaviour 
in Postgres? Can a CHECK constraint does that? Will a trigger with 
SELECT count(*) ... WHERE f IS NULL be too slow if the table is large?

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dave