weird results from trivial SELECT statement

Greetings,
I've got a Postgresql-8.4.x instance with a bunch of tables taht have
a text column (called 'active') that can contain any one of the
following values:
NULL
'disabled'
<some other text string>

When I run the following query, it seems to ignore NULL values:
SELECT * FROM mytbl WHERE active!='disabled'

and only returns rows where active!='disabled' AND active IS NOT NULL.
Is postgresql implicitly assuming that I want non-NULL values?

I can provide additional information, if requested.

thanks!