On Android with the new OS and newest version of Spoitify there is no way to offline your playlist to an SD card. This is a problem for people like me who have large playlist we want offline.
After some hunting I found this great procedure.
1. Uninstall Spotify and check that the Spotify folder in sdcard/Android/data has been removed.
2. Install the old version of Spotify from here
http://www.mediafire.com/?64njb8peymtkawn
3. You going to have go into Settings -> security and check Allow installation of non-Market apps
4. Download the app and install
5. Start the app and don’t log in (if you are logged in auto, log out)
6. Press the menu button and select the SD storage option.
7. Change the standard location to /mnt/extSdCard and then log in
8. Select a single playlist to download/offline play and let it finish downloading before you go to the next step.
9. Go to the Play Store and update the Spotify app to the newest version. It will save all cached music to the external SD card now until you uninstall the app provided you did everything correctly.