So far, this is only happening on cluster one (Cartman) but it's coming to all the clusters in the near future. I know that several people on my friend-of list have old journals which they have deleted, and they might not be expecting them to be purged since LJ has done that so infrequently in the past. Well, they're doing it now, so if you have an old journal which you have deleted as a substitute for making all the entries "friends only," and you want to keep it, this is the time to un-delete it, at least until the process is over.
I suspect that purging might take place more frequently in the future, as there is a group of users (probably small, but certainly vocal) who have been complaining about long-deleted journals taking up space on the servers (not a serious problem yet, as far as I can see) and, particularly, about user names which are unavailable because they already belong to someone with a never-used or deleted journal. Personally, I hear the sound of a can of worms being opened, but apparently Brad intends to forge ahead with this project. I hope the flames don't get too high.