I have the latest update of Spotify on an iPHone 4s, iOS 5.1 (not 5.1.1). I do not believe this was happening prior to the last update and I'm not sure where to send bug reports or if Spotify even takes them.
Anyway, if I turn off the screen while listening to Spotify then want to look at the lock screen and see what song is playing it will display the current song but if I look down again after the track has changed it still displays the title of the track that was playing the first time. If I unlock the screen then lock it back and check again it will display the correct song. But it will only display that track's info even when I'm further into the playlist.
In other words if I look at the lock screen during track 1 then lock the screen again and look back once I've progressed to track 3, it still displays track 1. Unlocking and relocking the screen refreshes the data to the current track but it remains stuck there until the next unlock.
While I'm here... any plans to allow us to rearrange playlists? I like my stuff to be better organized than it is. Folders would be great.