Odd Null Behavior

I am writing an upsert proc that should detect the change in state for a record. The change in state happens when a particular date field (default null) is populated. However, I can not get a record set that detects the changes properly.

Here is an example
set ANSI_NULLS on
go
create table #t1
(
ID int,
DateField datetime
)

create table #t2
(
ID int,
DateField datetime
)

insert into #t1 (ID, DateField) values (1, '7/20/2006')
insert into #t2 (ID, DateFIeld) values (1, null)

select * from #t1 join #t2 on #t1.ID = #t2.ID where #t1.DateField <> #t2.DateField

drop table #t1
drop table #t2

The select should return a record because NULL does not equal '7/20/2006' but it doesn't.
What am I missing?

Thanks in advance.