The MySQL reference (http://dev.mysql.com/doc/refman/5.0/en/create-index.html) says:
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index permits multiple NULL values for columns that can contain NULL.

According to the above a look-up into a unique index over a nullable column may return several records if the key value is null.
However iMariaDB 5.1/5.2/5.3/5.5 always returns not more than one row when using a null key to perform a look-up into an unique hash index. This can be seen from the following:

MariaDB [test]> CREATE TABLE t1
-> (
-> pk INT PRIMARY KEY,
-> val INT,
-> UNIQUE KEY USING HASH(val)
-> ) ENGINE=MEMORY;
Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> INSERT INTO t1 VALUES (1, NULL);
Query OK, 1 row affected (0.00 sec)

MariaDB [test]> INSERT INTO t1 VALUES (2, NULL);
Query OK, 1 row affected (0.00 sec)

MariaDB [test]> INSERT INTO t1 VALUES (3, 1);
Query OK, 1 row affected (0.00 sec)

MariaDB [test]> INSERT INTO t1 VALUES (4, NULL);
Query OK, 1 row affected (0.00 sec)

MariaDB [test]> EXPLAIN SELECT * FROM t1 WHERE val IS NULL;
--------------------------------------------------------------------+

--------------------------------------------------------------------+

--------------------------------------------------------------------+
1 row in set (0.01 sec)

MariaDB [test]> SELECT * FROM t1 WHERE val IS NULL;
--------+

--------+

--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM t1;
--------+

--------+

--------+
4 rows in set (0.00 sec)

(see also http://bugs.mysql.com/bug.php?id=44771)