The question got me curious enough to head over to wikipedia to learn a little more about the history of thieve's cant, and from there to several sites discussing the language in greater detail. It may be worth your time to do a little exploring of your own as you and your DM seek an answer that works for your table.
In the meantime, thieve's cant was a large collection of slang meant to obscure conversation from those outside the criminal underworld that employed it. As such, it stands to reason different formal languages would have different thieve's cants in a fantasy world. Also, given the generally fluid nature of language, there could be regional differences in thieve's cant that mirror regional dialects of the formal language. It's certainly possible (even likely) that social groups as small as a guild would add their own words to the language. Rival thieve's guilds, for instance, may like to invent certain phrases for discussing one another. However, I doubt it would be complex enough to constitute an entirely different language; just enough to prevent specific, relevant concepts from being understood by rivals.
That said, I'm sure the intent of the developers for game purposes is as Saelorn describes, a unified dialect understood by most/all members of the criminal underworld.
