When Steve Jobs unveiled Leopard a few months back, he said that he was leaving some stuff out, some “Top Secret” information kept that way so that Microsoft wouldn’t have the chance to copy it so soon. Most people doubted that this was the real reason; even if Microsoft will copy the hidden features (which they probably will, eventually), it would have been far too late for them to have incorporated the changes into Vista, and there will be way more than enough time after Leopard’s final release for them to still copy the features in the next version of Windows a few years down the road.

So why was some stuff top secret? Probably most of it has to do with upcoming product releases that Apple wants to keep secret for the time being, including the iPhone and some aspects of iTV, and perhaps even the rumored fullscreen iPod. Releasing some OS details may have given away too much about these items.

Other reasons could include Jobs keeping some juicy stuff saved for the January MacWorld Expo, a new strategy for keeping people interested in a product by stringing out descriptions over time (like with the iTV), or even because the features are unfinished and might not look good enough to release yet.

That last reason may be the one behind keeping the new OS’ interface changes secret. One of the rumored changes in Leopard is the Finder; many have hoped that Apple would “FtFF” (“Fix the Fracking Finder,” so to speak), and perhaps overhaul the overall appearance of the OS. There have been subtle hints about this–new styles applied to iTunes, and small imperfections of the GUI in developer releases of Leopard.

But today there is a rumor that reveals a little bit more: that the UI will have an all new name, to replace the “Aqua” moniker given it from the start. The new name is supposedly “Illuminous.” No details on the new look except for a report that the latest build of the OS incorporates more “black gloss” than might be expected. Though how black gloss might go along with the idea of illumination is anyone’s guess, and suggests that these rumors are either partially or fully untrue or that we’re really missing the big picture here.

Whatever the case, it is probably true that Apple has two lines of OS releases: one that is going out to developers, and one that is kept in-house, which has the “secret” features. The developers’ version is likely to have a certain amount of placeholder stuff (explaining some of the graphic rough edges), with only the parts of the secret stuff which are absolutely necessary to build into the OS being incorporated until Jobs is ready to spill some more beans.