There's a book called 'The Design Patterns Smalltalk Companion" by
Alpert, Brown and Woolf (ISBN 0-201-18462-1) that devotes a chapter to
your Decorator pattern pp 161-177. The book is interesting in a couple
of regards, basically it 'translates' all of the design patterns into
Smalltalk. My favorite bits have to do with the fact that a lot of the
"patterns" are part of  the language Smalltalk itself, and therefore
aren't something that need to be formalized as they are in other
languages/books. You'll notice that the book is much smaller than the
gang of four book. Personally I'm not a patterns type of guy, but I
can see how others might find it useful.