Notch said in a fairly recent interview (PC Gamer one, I believe) that he was considering following the Guild Wars 1 model for the multiverse, with stations and such being persistent, but the rest of the universe being instanced to your "party," basically. Has a lot of people on the official forums up in arms.
I think that the best, happy medium would be to have SP and player-hosted MP, like in Minecraft, available to people without a sub. Then you'd have a fully persistent, single-shard "multiverse" for a sub. Keeps just about everybody happy, I feel. The main argument in favor of the "sharded" or "instanced" model seems to be that solo players or small groups would get dominated by larger groups. Now, I don't feel that will be the case, as the universe is purportedly massive (with 100k+ star systems, I think it'd be pretty hard to even get found if you didn't want to), but I can empathize with people who just want to play with their friends and be left alone.
I still feel that anything less than a single persistent universe would really be killing the potential for the game. Things like economies, player government, all-out wars, and whatnot simply can't develop in that kind of instanced system, and that would be a real shame. It would also seriously hamper player-to-player interaction, in my mind, as well as really killing the "exploration" atmosphere.
Still, we won't know more until we know more. I personally hope Notch doesn't go the instanced route, but other people might feel differently. Just worthy of discussion, I think.