This is a Stability Release, which means it does not have any new features, it only contains fixes, to bugs discovered in the 1.0.0 release.
1.0.1 is available as a Full distribution, which contains all Joomla! files and a Patch distribution which contains only the files that have been changed by the Stability work conducted.
To update from Joomla! 1.0.0, all you have to do is simply overwrite your files with either the Full Package distribution, or the Patch Package distribution.
For those converting from Mambo please read the instructions these Migration instructions.
Thank you to the community for their assistance in helping us mae Joomla 1.0.x more stable and our apologies for missing the target release time.
Rey Gigataras [stingrey]
Joomla! Software Coding and Design
Stability Team Leader