These all work too:
Where [fieldname] = True
Where [fieldname] <> False
Where [fieldname] = Yes
Where [fieldname] <> No
Where [fieldname] = False
Where [fieldname] <> True
Where [fieldname] = No
Where [fieldname] <>Yes
Where [fieldname]
The last one works for true in sql because it resolves to True or False just like the other expressions.
In the design Access 2007 displays it as:
Where [fieldname] <> False
But it leaves the sql unchanged.