I know there are web services in Nintex but from what I understand they are predominately at the site collection level.
What web services are available at the farm level? Ideally, I would like to call a web service to get the list of tasks a user has available or call a web service to get the list of requests a user has issued.
One option would be to use the stored procedures available in the Nintex Content Database. For example rptWorkflowTasks returns all workflow tasks, possibly filtered for user, web, or site. There are also helpful procedures for information like completed workflows, workflows in progress etc.