Stephen, like Kenny said, Smart Lists are not accessible via API (when you think about the complexity and computation time that could potentially go into a SL, and the expectation that an API call will get the freshest data, it's understandable that this hasn't/yet been enabled).
A common workaround is adding to a static list via a batch campaign and then using the Get Leads by List ID method.
But if you are desperate to get Smart Lists as-is, you could think about using SL subscriptions and then processing the report with an email-to-database app. Something to think about. For a current client, I'm wondering if their client (the owners of the Mkto instance) might find setting up an SL subscription easier than giving us API access, since they seem very touchy and the whole SL-to-static might be a no-go.
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