This is handy. SharePoint helpfully populates the meta data with the GUID of the list and the ID of the item a WF instance is associated with. These are stored in "ows_WorkflowListId" and "ows_WorkflowItemId" fields on the task list item.

An example of using this is from an InfoPath form in the form load code behind.

SPListItem currentListItem = SPContext.Current.ListItem;

if (currentListItem != null)

{

  object associatedWfListId = currentListItem["ows_WorkflowListId"];

  object associatedWfItemId = currentListItem["ows_WorkflowItemId"];

  if (associatedWfItemId != null && associatedWfListId != null)

  {

    SPListItem item = web.Lists.GetList(new Guid(associatedWfListId.ToString()), false).GetItemById(int.Parse(associatedWfItemId.ToString()));

    // THE ABOVE ITEM IS THE ASSOCIATED LIST ITEM

  }

}