OS X doesn't really need desktop rebuilding in the OS 9 sense.
But when you have problems with the Finder and file and application associations (rebuilding the desktop was the solution to this problem in OS 9), you can fix this by doing the following:
1) Open up your ~/Library/Preferences folder (this is the Library folder inside your home folder).
2) Delete the files "LSApplications", "LSSchemes", and "LSClaimedTypes". (Don't worry, they will be recreated.)
3) Restart your computer.
Note: This will kill all your file/application associations. That is, if you have SimpleText open all of your text files (instead of the default TextEdit), they will all be reverted to the default applications association (which is TextEdit in this case).