If you double-click on any of the Input or Output nodes inside a subflow it will open the Edit Properties dialog (the same one if you click the 'edit properties' button in the toolbar.
As with other nodes, you can go to the 'Appearance' tab and set labels for the inputs and outputs.
These are the labels that appear when you hover the mouse over the input/output ports of the node in the main workspace.
You do not currently get to see these labels when editing the subflow itself - that isn't something I'd thought of but will look at adding as it makes sense to do so.
You can use node.status() in nodes within the subflow. The status event can be handled by a Status node however the status event will not appear against the subflow instance node - in otherwords, you won't get the text appearing below subflow instance node.
As @colin mentioned, we just released the next beta of 0.20 which adds some new subflow capabilities, including better handling of status.
