After updating to 4.2GM, I noticed that Spotify would stop playing when the phone locked or was put into standby. However, after restarting my Phone it started working OK again. It seems to be related to AirPlay - it looks like Airplay will occasionally mess up and that prevents Spotify from replacing the iPod app in the multi-tasking bar. Once that happens, Spotify stops playing when the iPhone goes to sleep. So, it looks like it's either a bug in Spotify or in the iPhone GM.