How can I force the built-in Music App in Android to refresh my list of playlists. Even after I delete one from my SD card, the playlist will still appear in my list of playlists.
Go to settings, applications, manage applications, [menu button], filter, all
Find the Media Storage app, and clear data.
This deletes everything! artists, albums, playlists. (data still on card)
Plug in, mount usb. Eject. Go back into music, and wait (a lot if you have more than a few tracks.)
Note: If the music app shows no changes, try force stopping the Media Storage app. The app showed no cache or data to clear, but force stopping it then opening the Music app worked for me.
I have a Moto Droid, and believe it or not, I had to do a factory reset on the phone (Settings -> Privacy -> Factory Data Reset) to get rid of the playlists that I erroneously put on the SD card.
After the phone rebooted, it reactivated my service and restored the apps I had previously downloaded from the Android Market. The customizations I had made to my home screen needed to be redone, and the device needed to re-fetch all of my Google data (contacts, calendar, etc.)
They really need to come up with a better solution to that, but thankfully, I hadn't done much with the phone, so I didn't have many things that I needed to setup again.
There is an application that deletes unused playlists from the database. See http://roryok.com/blog/2010/07/23/clearing-out-deleted-playlists-in-android/