Its quite interesting what smalltalk ANSI draft says about it:

----
3.4.6.6 Array Literals
An array literal is a sequenced collection with numeric keys which may
contain any number of other
literals.
<array literal> ::= '#(' <array element>* ')'
<array element> ::= <literal> | identifier
The value of an array literal is an object that implements the
<sequencedReadableCollection>
protocol. The elements of an array literal can consist of any
combination of literal forms. If an
identifier appears as an <array element> and it is one of the reserved
identifiers nil, true or false
the value of the corresponding element of the collection is the value
of that reserved identifier. The
meaning is undefined if any other identifier is used as an <array
element>. If an <array literal> has
no <array element>clauses the collection has no elements.
----

So, as i understood, if meaning for any other identifier, except nil,
true, false is undefined - then parser should produce the syntax
error, isnt?

Conclusion: Squeak doesn't follows ANSI standard in this area :)

Next interesting thing in following:

3.4.7 Reserved Identifiers

<snip>

Implementations may define other identifiers with bindings that have
implementation specified
semantics. Any such identifier must be bound in the extension scope of
the program.. An explicit
definition of such an identifier in any scope supersedes the
implementation provided binding.
-----

as i understood, standard does not says, that given set of reserved
identifiers is final?
So, one may indeed , introduce extra names such as null, epsilon,
undefined and it still will be smalltalk!
But we all know that system dictionary is much better place for them,
unless a new name provides a completely different semantics, like
thread local storage.

-- 
Best regards,
Igor Stasenko AKA sig.