I had that issue when I updated the OS to 5.01 a couple of months ago. A reboot didn't help, so I deleted all 13 GB of music from the phone and put it back on with iTunes. Every song I have played since loading them back onto the phone has played, even the ones I had problems with just after the OS update.
Someone else may know of an easier, less time consuming, fix.
I too hope for him, but usually unsyncing and rebooting the iPhone and then resyncing should cure the trouble. I had this thing with my iPhone 4S before when i was trying to sync all my computer songs into the iPhone (over 1000 songs) some of them went grey, i unsync everything rebooted the iPhone and sync back in again and they work. Couldn't know what was causing it though.