In the commit tool, double clicking on any of the items in the commit list currently simply toggles the checkbox that allows you to select whether the item (file or subrepo) will be included in the commit or not.
I don't think that this is very useful (since you already have the checkbox for that!).
Instead I think that a more useful behaviour would be:
For subrepos: The subrepository history browser should be open on a new tab (or if it is already open, the corresponding tab should be selected and the working copy "revision" should be shown). This is the same action as if you had clicked on the "open..." link that is shown on the subrepo diff.
For files: The visual diff tool should be open, showing the change that would be committed.