Download Astro file manager from the Android Market for free. Use Astro to view the folders on your phone/sd card. Rename any folders that you want to "hide" by putting a period in front of the name. For instance, change "Music" to ".Music", "Pictures" to ".Pictures", etc. All of the media files in that folder will disappear from your music players, gallery, etc. but they will still be on the phone and accessible through Astro. If you want to make those files accessible in other apps again, just remove the period from the beginning of the folder name.