I saw this in another thread : http://community.spotify.com/t5/Mobile-iOS-iPhone-
Perhaps there is a partial fix coming? It'd be nice to know an approximately timeline, 1 month, 3 months, etc...
This has happened to me twice today and at least twice a week for the past month. Really annoying as it takes hours to try and re-sync everything.
They've just released a new version on the app on the iTunes store, and the release notes specifically state that this fixes the offline synching issues. I'm gonna try it out... although it's only happened to me twice since Jan 2012.