You mean you downloaded Darwin 8.0.1 from the Apple Developer's Connection site, and installed it over Mac OS X?
If so, I'm afraid there is no easy way to "uninstall" or "go back" to Mac OS X. It all depends on how you installed Darwin -- did you install it to a separate partition or drive, or did you install it on the same drive you had OS X installed? If you installed Darwin on the same drive as Mac OS X, then you have Darwin only on that drive now -- Darwin and Mac OS X cannot exist simultaneously on the same partition
There is no "Darwin for Mac OS X" -- there's Darwin that's compatible with PPC processors, and Darwin that's compatible with Intel processors. Darwin is not "for" OS X... Darwin is a complete operating system itself.