If like me you found Windows Media Player 11 [WMP] to be complete crud & iTunes to be a necessary evil. Then you may lament the lack of an option to rescan your music library under iTunes. WMP, Banshee & Winamp, to name a few, all have this feature.
With iTunes, however, you have to either manually add each & every new folder or drag & drop your music files into the iTunes window itself.
You could use a third party app, but by far the most eloquent solution is to paste new content into the Music-> iTunes-> iTunes Media-> Automatically Add to iTunes folder.