I want to use REST to extract a list of leads from Marketo.
Can anyone give me some direction with this.
You cannot retrieve from Smart Lists at this time, only Static Lists (i.e. "Lists").
The Marketo Bulk Extract API, which begins release this very month, will be able to retrieve based on Smart List membership. However, your instance may not be enabled for SL retrieval so soon (the feature requires that your back end be upgraded first, which is being done on an as-needed basis based on volume).
O.K next question...
If i wanted to extract a list of leads who have changed their data value with in a certain time period. How can this be done using a REST API?
Have you read the developer docs? This should be clear from reading about the Activities - Lead Changes API endpoint.
Is this Smart List Leads API now available? I just tried downloading leads using this API and was expecting it to return some leads, but API response was empty.
Has anyone trailed this one with success?
It wouldn't be accessed that way, even if it were available in your instance -- there's no such signature.
The docs and endpoint definition are at http://developers.marketo.com/rest-api/bulk-extract/bulk-lead-extract/
Yeah you can programmatically access smart lists. The general flow is to create the bulk job, grab the job id, enqueue the job, and export when the job status hits complete. I have some code that I can send over if it would be helpful, just let me know.