There are only 15 slots for icon overlays available on Windows. And some of them are already used by Windows itself, for example shortcuts have an overlay (small black arrow), shared folders (hand), offline files and one or two more. On Vista, there's at least one more used to indicate programs which require elevated privileges (small shield).
That means for other programs, there are around 10 overlay slots available, which must be shared among all other applications/shell extensions which use overlays.
We don't want to waste a slot for the least important information. If you want to distinguish unversioned and ignored files in the explorer, switch to the explorer's detailed view and activate the column SVN Status. Then you will see the difference.
See also: The overlay icons appear, but not all of them!