Had my htc incredible for 10 days now and been hugely vexed with the music player and synching iTunes. Used a couple of apps and was able to sync everything (cover art sync is weirdly hit or miss though), but pressing a playlist returned the message that the playlist was empty. Any other view (artist, song, album, genre) displayed all of the info, and the songs played fine when you selected one. Trolled a bunch of forums and sent notes to app developers, but no fix.
I found one. It's a little work, but not much. The problem is that some hidden feature in the way android is implemented in the incredible causes the playlist feature to have trouble with capital letters. [I felt like I accidently clicked on an easter egg when I figured this out]
Here's the fix then. You have to manually delete all the playlists in the incredible, which requires "x"ing each one. After that, mount the sd card and manually delete all the music files and folders, and you must delete any app stored data as well (e.g., userprefs, ratings, xml, etc.). Reinstall your sync program if you have to (I didn't, using Syncr). Then unmount the phone (very important! do not just pull the USB plug!).
Next, open iTunes and click on each playlist, changing all upper case letters to lower case. Then mount the phone back on the PC and sync. When it's done, unmount the phone (don't just pull the plug) and wait for the phone to remount the sd card for its own use. Open the music app and you may or may not see your old playlists, albums etc. for a few seconds, but they'll go away and a song will show up. Play it for a few seconds, pause it and set the phone down, go get a sammich or nice piece of fruit and give the phone time to fully rescan the sd card. Go to playlists, and they'll all work.
Note - you only have to remove capital letters from the iTunes playlists. Capital letters everywhere else are fine.
Hope it works for you like it did for me.