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
}
}