Just a datapoint:
SQL*Plus: Release 10.1.0.3.0 - Production on Tue Jan 30 15:15:49 2007
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Connected to:
Oracle Database 10g Enterprise Edition Release 10.1.0.3.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> select * from dual where '' IS NULL;
D
-
X
SQL> select * from dual where '' = NULL;
no rows selected
-j