Album art not syncing to iPhone for playlists
Denis Ahrens mentioned something about that in the last episode of mobileMacs (timecode).

For those of you who don't know German: He said that all of his music is lossless and that iTunes recently learned how to encode these files to 256kbit/s AAC when syncing his music to his iPhone. When he switched this on, he experienced the same problem that you have: No cover art at all. He explained that some people seem to get the cover art and some don't.

If I understood him right (even for native speakers, he talks very fast and the context isn't always clear) he says that the parts of your library that get converted from lossless while transferring are missing the cover art while the tracks/albus/files that you convert explicitly before syncing get the cover art – but as I said, I'm not sure about that.

I have neither iOS 6 nor lossless music so I can't test this theory myself, I can only pass on what I heard; I hope it helps. (By the way, Denis concludes that he hasn't found a workaround yet and advises to wait for the fix.)