In iTunes 8, there's no longer a
File > Import... item in the menu.
If you want to import a library XML file — after re-installing your system, or when moving the library from another computer, for example — you can use the
File > Library > Import Playlist... menu item.
It's confusing, since there is a
File > Library > Export Library... command, and it's not obvious that you export a "Library" but import it back as a "Playlist".