I think the transition plan will be like this:
-- pretty soon, no new accounts will be able to be created on cluster 0. if you don't have $LJ::DEFAULT_CLUSTER defined, it'll default to 1. if you don't have cluster 1 settings defined, it'll assume you only have one database server and use it.
-- later, journal/comment posting code will be removed for cluster 0, but it'll all be readable still.
-- finally, reading code will be removed.
So, if you're running an LJ site with users on cluster 0 still, start moving users! (hopefully scsi can help people with this transition, since he's in the middle of it right now....)