If you don't like the horizontal stripes that OS X puts behind most things (menus, title bars, etc), there are now some "unlined" themes and an easy-to-use theme switcher for OS X. If you're interested in these, I'd suggest you grab them quickly, as Apple legal may not allow them to live very long.
To use, simply download the MetamorphX theme switcher, and then grab your choice of the unlined themes, launch MetamorphX and choose your stripeless theme.
I'm not sure if Apple will provide any 'official' themes for OS X or not, but here's a way to remove the stripes now if they bother you. Get 'em while you can!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010530122234120