Martin Fowler gives a great talk about the style of domain specific languages, and clearly covers all the main points about the advantages and disadvantages of those styles. This guy is always worth watching, he's very good at summarizing all the sides of an issue in a very unbiased and informative manner. He doesn't specifically mention Smalltalk, but every time he says Lisp, you can just pretend he said Smalltalk, it all applies.
Here's the Smalltalk version of his sample code, in a Smalltalk DSL for comparison.
(Mapping for: #SVCL type: ServiceCall) extract: 4 to: 18 for: 'customer_name'; extract: 19 to: 23 for: 'customer_id'; extract: 24 to: 27 for: 'call_type_code'; extract: 4 to: 18 for: 'date_of_call_string'. (Mapping for: #USAGE type: Usage) extract: 9 to: 22 for: 'customer_name'; extract: 4 to: 8 for: 'customer_ID'; extract: 30 to: 30 for: 'cycle'; extract: 31 to: 36 for: 'read_date'.
Simple, clean, no XML necessary, just objects and messages!