While on two flights yesterday I attempted to use Spotify, with the app obviously in Flight Mode. I also have offline mode, don't sync over 3G etc enabled First thing it tries to do? Connect to the net.
The app couldn't be used at all because it claimed my playlists aren't available offline (what?).
If I set my WP8 Spotify to Offline mode, I get the same error message on my playlists, but this is because my playlists are "in the cloud" and WP8 Spotify can not access the phones' music library so it has nothing to play.
For offline mode to work, the important thing is to have previously made your playlists available offline (i.e. downloaded the tracks to the phone). Had you made the playlists available offline before you made your your trip?