String Comparison and NULL

[seijin(at)gmail(dot)com]

Hi,

I'm fairly new to PG and databases in general so this may very well be
a problem in my thought process.

If I have a simple table with an ID (integer) and Animal (text) like
this...

1 Dog
2 Cat
3 NULL
4 Horse
5 Pig
6 Cat
7 Cat

... and I do something like "select id where animal <> 'Cat';"  then
shouldn't 1, 3, 4 and 5 be picked?  As it is I only get 1, 4 and 5.
NULL is not 'Cat'.  I realize that if I were testing for NULL itself I
would use IS or IS NOT but this...?  I'm a little confused.

Thanks!