UUID can be written to row when passed as parameter/argument to a PL/pgSQL function. Feature or bug?

If I pass the hex string representation of a UUID to a PL/pgSQL function as a varchar, that value cannot be used
directlywhen writing to a row whose column data type is "uuid", in Postgres 9.0.x. Normally Postgres automatically
convertsa hex string to a 128-bit UUID value and back again.  

Is not doing so in a function a bug?

Example follows below.

(1) Create a simple table with one column of type "uuid".

--->
CREATE TABLE uuid_tbl_
(
  uuid_col_ uuid NOT NULL
)
WITH (
  OIDS=FALSE
);
<---

(2) Create this function.

--->
CREATE OR REPLACE FUNCTION uuid_write_( varchar ) RETURNS boolean AS $$

BEGIN
    INSERT INTO uuid_tbl_ ( uuid_col_ )
    VALUES ( $1 );
    RETURN True;
END;

$$ LANGUAGE plpgsql;
<----

(3) Call this function:
select uuid_write_('34A94C40-453A-4A30-9404-128121E76570');

Note the error:
---->
ERROR:  column "uuid_col_" is of type uuid but expression is of type character varying
LINE 2:  VALUES ( $1 )
                  ^
HINT:  You will need to rewrite or cast the expression.
QUERY:  INSERT INTO uuid_tbl_ ( uuid_col_ )
    VALUES ( $1 )
CONTEXT:  PL/pgSQL function "uuid_write_" line 3 at SQL statement

********** Error **********

ERROR: column "uuid_col_" is of type uuid but expression is of type character varying
SQL state: 42804
Hint: You will need to rewrite or cast the expression.
Context: PL/pgSQL function "uuid_write_" line 3 at SQL statement
<------

(4) Change the function by assigning the passed hex string to a variable named 'uuid_arg' and declared to be of type
"uuid",then write that variable to the row instead of the argument.  

----->
CREATE OR REPLACE FUNCTION uuid_write_( varchar ) RETURNS boolean AS $$

DECLARE
    uuid_arg uuid;
BEGIN
    uuid_arg := $1;
    INSERT INTO uuid_tbl_ ( uuid_col_ )
    VALUES ( uuid_arg );
    RETURN True;
END;

$$ LANGUAGE plpgsql;
<-----

(5) Run the same line calling this function:

select uuid_write_('34A94C40-453A-4A30-9404-128121E76570');

Note the success of this workaround.

My blog post on this issue:
http://crafted-software.blogspot.com/2011/04/passing-uuid-value-to-function-in.html

--Basil Bourque