<p>You could have relative and absolute types of URL... Would that make things simpler, or more complicated? For that matter, you could implement as much of the URI syntax as made sense in the type system. ftp might make sense; file probably wouldn't.
In the context of the WYSIWYG editor, which is where I was focussed for the bug report, maybe a dot slash syntax could indicate a relative URL, or maybe there could be a third radio button in the dialog: internal link, absolute external link, relative link.